I have been playing hardcore mode on a Minecraft world in 20w51a for about ~40 hours when I saw the latest snapshot had changed to 1.16.5-rc1 and immediately backed up my world so I could optimize it. After Optimizing it I played for another hour or so before I discovered my tagged zombie "entitled" was his name had despawned/disappeared. I had found him and used a nametag on him because he spawned in with a full set of enchanted diamond armor. Once I found that he had despawned I created a new save world with my recently created backup and checked to confirm he had despawned after I had optimized the world. I then made another copy of the world and optimized it another time to see if this was a repeatable bug and it despawned entitled once again. Naturally I was most concerned with my rare mob but several other types of entities disappeared/despawned as well. Entities that despawned include: Armor stands, tagged enderman in a boat, a boat, villagers and a nametagged zombie. That concludes my report I will further describe each entity that despawned below.

The armor stands that despawned include: a armor stand with enchanted leather helmet, chestplate, leggings and boots. a armor stand like the last but gold, another like the first but chainmail and a fourth with enchanted iron chestplate, leggings and boots.

The enderman that despawned was in a boat which also despawned, he was nametagged "dumpy".

The three villagers that despawned were all originally zombie villagers that had naturally spawned and then were cured via weakness potion and golden apple. One was cured two times in this fashion.

A snowman may have despawned as well.