Mobs (named & unnamed) on minecarts tend to get off when reloaded either through nether portals or going back around those chunks, no despawns yet. Doesn't always happen but after a couple times it becomes inevitable, and usually happens when mob's hitbox clashes with that of blocks around it, ei. ghasts in minecarts encased in a 3x3 glass hallow tube. I remember the same issue existing in the first couple versions of bedrock but being fixed not long after.