I occasionally have flying dragons disappear entirely- once an alpine that flew away from me before poofing, and once earlier today a Royal Red that popped into existence well within render distance, flew across my screen, and then disappeared again. Both times I initially assumed they had flown out of render distance, but on further reflection they were much too close, and chasing them proved fruitless- they had vanished completely.

I was using 1.2.10 when the alpine did it, and 1.2.11 when the RR did.

My son has had the same thing happen to him 6 times in the last 10 days.

This has happened to me at least twice with tamed alpines in 1.16.4. One of the vanished while I was riding it.