when the armadillo is in a minecart and gets 16 blocks away from the player the armadillo de-spawns. This behavior is consistent in the nether.
MCPE-188539
Armadillo despawns out of minecart
Resolved
Invalid
0
All naturally-spawned mobs instantly despawn when far from the player, unless they have been made persistent by using a name-tag on them, or interacting by shearing, taming, tempting with food, or breeding. The instant despawn distance is based on world simulation distance. You can read more about this on minecraft.wiki.