When you try to get an empty minecart to travel through a nether portal, it works perfectly fine, but it doesn't work if there is a villager inside the minecart. I haven't tested this with any other mobs yet. In my situation, I am transporting villagers from the nether back to the overworld.
Linked issues
is duplicated by 8
MCPE-89480
Nether portals
Resolved
MCPE-122087
Villager in minecart portal bug
Resolved
MCPE-134554
multiple entities can't teleport at once in the same nether portal
Resolved
MCPE-169037
Minecarts with passengers fail to go through end gateways
Resolved
MCPE-181884
Entities with passengers cannot use Nether Portals, End Portals and End Gateways
Resolved
3 more links
relates to 2
Attachments
Comments 4
relates to MC-67
As Skaijie mentioned, this relates to MC-67 which has recently been fixed in Java, so it can also be considered vanilla-parity.
I don't think that's a bug, that's how it's been for a while.