When I try to ride a mob that is constantly modified with the /entitydata command, the game teleports me and the entity to x=0 y=0 z=0. It doesn't metter what tag you're changing, the game teleports you there.
MC-67771
Weird minecraft issue
Resolved
Won't Fix
0
Please do not report issues related to NBT tags as bugs. If changing NBT via commands does not have the expected effect, that's always considered "working as intended".