So, me and my friend have a server on aternos and in our iron farm, we had 3 renamed zombies to speed up the iron production, but sometimes all our zombies disappear. There is no direct sunlight shining on them, there is a roof made of bottom slabs and we also sometimes enter the End and nether if it helps resolve the bug.

PS: also happened in 1.14.2 and 1.14.4