An endermite in a minecart with a name tag keeps despawning and I dont know why

I am having the same issue with iron golems. The first one disappeared and I thought that maybe it escaped so I created a new one but with a name tag and I put fences in him but still he despawned. I made a third one but I'm keeping him with lead and that fixed my issue but it's still annoying.

duplicates MCPE-21416