score:8
I have travelled on the ΓBB Nightjet from Germany to Switzerland, and can confirm that indeed they require a printed ticket.
I suspect that the reason is that on long international trips the ticket may be checked multiple times, as crews are changed, and with a paper print the first conductor can just stamp it, thus signalling to subsequent ticket checkers that the ticket is ok.
And on night trains it is common for the tickets to be kept by the steward, so that when the conductor comes to check the tickets the passengers do not need to be woken up.