Happened to me as well right now.
1.15.2 unmodded singleplayer
Zombie was nametagged, had armor (including helmet) and a sword.
Despawned on me while I was in the same chunk.
~Edit: That mob also was sitting in a boat. When it got back into view, Zombert was gone and boat still there.
Contrary to other reports, I did NOT get the nametag back.
Attached forced crash report
Don't know if its related but (Hopper-)Minecarts sometimes lose their momentum when the server is shutting down or the chunk unloads. Cannot reliably reproduce. Version 1.16.5