Arius' death was it miraculous or was he poisoned?

Arius lived to be about 80 years old, so the idea that his sudden death was "miraculous" was probably wishful thinking by his opponents. It was certainly seen as an Act of God by both his two orthodox enemies Athanasius and Alexander. The following from a letter of Athanasius:

(Alexander prayed) "If Arius is brought to communion tomorrow, let me, Thy servant, depart, and destroy not the pious with the impious." When the Bishop (Alexander) had thus prayed, he retired with great anxiety, and a wonderful and extraordinary circumstance took place. . . Arius, in the language of Scripture, `falling headlong he burst asunder in the midst,' and immediately expired as he lay, and was deprived both of communion and of his life together. Such has been the end of Arius.

Could he have been poisoned instead? Perhaps but if so, the manner of his death was probably exaggerated. Socrates Scholasticus describes his death as follows:

It was then Saturday, and … going out of the imperial palace, attended by a crowd of Eusebian [Eusebius of Nicomedia, a semi-Arian bishop] partisans like guards, he [Arius] paraded proudly through the midst of the city, attracting the notice of all the people. As he approached the place called Constantine's Forum, where the column of porphyry is erected, a terror arising from the remorse of conscience seized Arius, and with the terror a violent relaxation of the bowels: he therefore inquired whether there was a convenient place near, and being directed to the back of Constantine's Forum, he hastened thither. Soon after a faintness came over him, and together with the evacuations his bowels protruded, followed by a copious hemorrhage, and the descent of the smaller intestines: moreover portions of his spleen and liver were brought off in the effusion of blood, so that he almost immediately died. The scene of this catastrophe still is shown at Constantinople, as I have said, behind the shambles in the colonnade: and by persons going by pointing the finger at the place, there is a perpetual remembrance preserved of this extraordinary kind of death

This and other reports need to be taken with a grain of salt. According to University of Michigan scholar Ellen Muehlberger:

Starting in the 360s CE , numerous late ancient sources began to report that Arius’ death was the result of explosive gastrointestinal problems he suffered in the city of Constantinople while he was attempting to negotiate admittance to the church there. Because of the graphic nature of the reports, most historians have expressed caution about their veracity; both the gruesome details of the story and its delayed appearance in the historical record have led scholars such as Williams to consign it to ‘the sphere of melodramatic semi-fiction’. 4 Thus the story remains curiously suspended: not entirely dismissed as a fabrication, but not taken as historically reliable either.

Arsenic may be the culprit

The OP asks "Have any historians or medical professionals ever explained what type of poison could have produced the death of Arius in such a gruesome manner?" The answer is yes. Arsenic is the leading candidate because it mimics extreme food poising. Assuming Arius died from violent bowel-emptying (exaggerated by Socrates and other accounts), arsenic could have been the agent.

Dioscorides, a Greek physician in the court of the Roman Emperor Nero, described arsenic as a poison in the first century. Its ideal properties for sinister uses included its lack of color, odor or taste when mixed in food or drink and its ubiquitous distribution in nature, which made it readily available to all classes of society. Symptoms of arsenic poisoning were difficult to detect, since they could mimic food poisoning and other common disorders. There could be no doubt about arsenic’s efficacy as a single large dose, which provoked violent abdominal cramping, diarrhea and vomiting, often followed by death from shock.

But Mullenberger advises us to consider the influence of myth-making here, both of the anti-Arian and pro-Arian variety. Anti-Arians related the story of his death as miraculous: the venomous heretic a just victim of his own spiritual poison. Pro-Arians, on the other hand saw the event as the result of a murder and martyrdom. But at 80, he may easily have died of food poisoning or a natural stomach ailment, with the narrative then exaggerated by friends and foes alike.

Not only did late ancient Christians regularly practise seeing the invisible alongside the visible, they also regularly sought to visualize the multiple temporalities that could be represented in a single space. Once established, the image of Arius dying in the midst of the Forum, near the porphyry column, for every eye to see, worked retroactively to define Constantine’s city as always having been the place where Arius died. Rather than there having been a historical awareness of the emergence of the legend of his death over time, the version of the tale told in Socrates’ history offered a past for Constantinople in which Arius’ death in 336 CE was continuously memorialized in the Forum from its founding in 337 CE onward.

Mullenberg's article has many footnotes but they are most useful for documenting similar, apparently exaggerated, narratives rather than deciding what particular poison might have been used. In terms of candidates for the supposed poison itself, arsenic is as good as any since it causes violent bowel emptying and was known to the ancient world, even if its effects are not as dramatic as described above.