Is what is impermanent always the cause of suffering?

If we look at tilakkhana: anicca dukkha anatta. Anicca is impermanent and as a condition for suffering (dukkha). What is impermanent or suffering without atta And they are sankhatta dhamma.

Your question: "is what is impermanent always the cause of suffering?"

Let's ask the same question in a different way: is there something that is impermanent that causes happiness?

Let's say the answer is Yes.

Does it last forever? No.

When it is gone, would you be unhappy or unsatisfied? Yes.

Therefore, that which is impermanent always (eventually) causes suffering.

Let's ask another question: is there something that is permanent which causes suffering?

What is permanent?

Even time and space that are conditioned on each other according to General Relativity is not permanent. Even subatomic particles that can be transformed into energy and energy back to matter, is not permanent.

Only Nirvana is permanent. And Nirvana does not cause suffering. It's the end of suffering.

Since all that is permanent does not cause suffering AND all that is impermanent (eventually) cause suffering, therefore, that which is impermanent is always the cause of suffering.

OP: There is something which is impermanent and causes suffering. When it is gone you will be happy. That something is for example disease. Therefore that which is impermanent is not always the cause of suffering.

Disease, ageing, bodily decay and misfortunes are unpleasant situations.

After the unpleasant situation is gone, you would be happy.

But can you be free from unpleasant situations forever? No. As long as you're in samsara, unpleasant situations can come back.

If you are free from one disease, you can still get another disease.

When any unpleasant situation comes back, would you be unhappy or unsatisfied? Yes.

Therefore, that which is impermanent, regardless of whether they are pleasant or unpleasant, always (eventually) cause suffering.

It's not that Impermanent is "the cause" of suffering. The cause of suffering is mismatch between "is" and "should", and the cause of mismatch is attachment to wrong idea that something "should" be this way, when in reality it is not that way and should not be that way. We just have a mistaken idea about something that does not match reality.

In the case of impermanency, we have an idea of a thing, an entity, we completely forget that an entity is just a temporary arrangement of its causes and conditions. These causes and conditions had a different configuration in the past and will have a different configuration in the future, but in the present they have this configuration. Our mind calls this configuration "a thing" but it's not a thing, it's just a point in time snapshot, that we called "a thing", as if static things truly existed (they do not).

So this idea of "a thing" is an illusion. If we hold on to it long enough, the reality will move ahead, but the illusion in our head stays the same. So we try to act, as if it was real, we try to stand on it, lean on it for support - but it cannot be reliable support.

Illusion can't serve as reliable support or reliable ground, only truth can.

And what is truth? The truth is, what we call things are in fact ephemeral arrangements. This can be a reliable support for peace. How can it be reliable support for peace? Because if we see things this way, we will not have a mismatch between "is" and "should", we always have "is" and "is". Without a "should" to attach to, we always in suchness, always in tathata. This suchness or tathata is the true Nirvana, it is unconditional, peaceful, and perfect. There's nothing missing in it, nothing extra, it is just as it is and should be. Great Perfection.

That's why Buddha said, "I found the ultimate, the other shore, peaceful, reliable - but this world delights in attachment."

And this is why Mahayana says that Enlightenment is ever-present and everyone is already Buddha, because everything is already "suchness", it's just that people have confusion about things.

"My question is : Is what is impermanent always the cause of suffering?"

No, what is impermanent is not the cause of dukkha, what is impermanent is dukkha.

Meaning, what is impermanent is unsatisfactory in its absolute sense; it's out of one's complete control, and even if pleasing, it's pleasantness has an expiration date, bound to change, e.g., into something unpleasant.

And what is the cause, the origin of dukkha?

“And what, bhikkhus, is the noble truth of the origin of suffering? It is this craving which leads to renewed existence, accompanied by delight and lust, seeking delight here and there; that is, craving for sensual pleasures, craving for existence, craving for extermination. This is called the noble truth of the origin of suffering.

-- SN 56.13

Thus, we seeks delight in the impermanent pleasures of the senses hoping to find final rest to suffering. However any rest found from suffering in what is impermanent is always temporary at best.

Therefore, desiring for things that are impermanent is not only bound to never bring ultimate satisfaction, but is also bound to constantly put oneself in danger of suffering, for one cannot make what is impermanent to be always and constantly pleasing forever.

“Bhikkhus, there are these five situations that are unobtainable by an ascetic or a brahmin, by a deva, Māra, or Brahmā, or by anyone in the world. What five? (1) ‘May what is subject to old age not grow old!’: this is a situation that is unobtainable by an ascetic or a brahmin, by a deva, Māra, or Brahmā, or by anyone in the world. (2) ‘May what is subject to illness not fall ill!’: this is a situation that is unobtainable by an ascetic … or by anyone in the world. (3) ‘May what is subject to death not die!’: this is a situation that is unobtainable by an ascetic … or by anyone in the world. (4) ‘May what is subject to destruction not be destroyed!’: this is a situation that is unobtainable by an ascetic … or by anyone in the world. (5) ‘May what is subject to loss not be lost!’: this is a situation that is unobtainable by an ascetic or a brahmin, by a deva, Māra, or Brahmā, or by anyone in the world.

-- AN 5.48

In conclusion, it is the desire for those impermanent things which is the culprit, since it's by destroying that desire (and not by destroying what is impermanent) that one puts a definite end to suffering.

Is what is impermanent always the cause of suffering?

It's not. The cause of suffering is your Identification with the Impermanent.

Getting angry or possessive causes suffering. Impermanence does not have to occur. Mere attachment is enough, which is why the 1st noble truth defines all suffering as attachment (upadana).

When Buddha asks "Is what is impermanent suffering or happiness?", he is essentially asking: "Can that which is impermanent bring lasting happiness or are impermanent things incapable of bringing lasting happiness?"

The different teaching in MN 115 about the Three Characteristics provides a clearer translation than the common translation of the Three Characteristics based in SN 22.59:

Here, Ānanda, a bhikkhu understands: ‘It is impossible, it cannot happen that a person possessing right view could treat any formation as permanent ― there is no such possibility.’ And he understands: ‘It is possible that an ordinary person might treat some formation as permanent ― there is such a possibility.’ He understands: ‘It is impossible, it cannot happen that a person possessing right view could treat any formation as happiness― there is no such possibility.’ And he understands: ‘It is possible that an ordinary person might treat some formation as happiness ― there is such a possibility.’ He understands: ‘It is impossible, it cannot happen that a person possessing right view could treat anything as self ― there is no such possibility.’ And he understands: ‘It is possible that an ordinary person might treat something as self ― there is such a possibility.’

MN 115