Two of us share a Wahoo KICKR, each running from our own accounts on an iPad. However, when one of us changes trainer difficulty (I like 100%, she likes 70%), it changes as well for the other. This suggests trainer difficulty is a parameter associated with the trainer, not the user. I think this is new, that a few months ago we were able to maintain independent trainer difficulties.
Obviously the solution is to set trainer difficulty at the start of each ride. However, I don’t think there’s a rational basis to assume trainer difficulty should be universal for a given trainer.