Client/Server weirdness with mounted entity

So, I have an strange issue. I'm spawning an entity, and making it mount another entity using mountEntity(). According to my debug output, both the client and the server are recognizing that the entity is mounted, and updating its position correctly. Except: whenever the server sends an entity update to the client, the riding entity flickers back to its starting position for one tick, before moving back to the entity it's riding on the next tick. (This repositioning is not just visual, the debug console output shows that its position is actually reverting for this one tick.)  Increasing the update rate for the entity makes it do this flicker more often (which is how I know it's the update from the server that's doing it). But, as I mentioned, the server has the entity's correct position, so I have no idea where it's getting the incorrect position that it's sending to the client. Any ideas? I am utterly baffled.