We’ve investigated this, and the problem appears to be caused by standing too close to the boss and pushing it. This causes positions to get out of sync between the client and server for the rest of the fight.
This isn’t a new bug in 0.9i, and being able to push enemies in Online mode is a general bug rather than specific to this boss. But, we will look into a workaround for the Emperor of Corpses specifically since this is particularly bad.
For now, try to avoid pushing the boss around. Sorry about that!