I had mobs name tagged in minecarts, but when I updated to version 1.38 they were gone.
I also had a spider jockey (both name tagged) and the skeleton disappeared.
Steps to reproduce
use previous version of game.
put mob in minecart,
save, and load 1.38
Mobs that disappeared in minecarts that were named
witch, pigman, the elder guardians were not named tagged.