When a zombie villager touches a minecart it just collides with it, it doesn't actually hop on the minecart. If you try to convert a normal villager that's in a minecart into a zombie, it'll just hope out of the minecart.
MC-167354
Zombie Villager Cannot Ride Minecarts.
Resolved
Cannot Reproduce
0
Both of these issues have been resolved in 1.15 pre-6 upon further testing.