There are not many interactions available between a standing sim and a sitting sim. One I’ve seen is the ‘Call Over’ if the sims are far away from each other. But I would think all of the interactions would be available, and the sitting sim would just stand up before performing the interaction. If you’re visiting a Sim and they sit down, you won’t be able to talk to them! You’ll have to sit down as well and see what options are available then. For example, if you both sit down in front of the TV there should be several things to talk about. But I think a standing sim should still be able to talk to a sitting sim. It seems like a bug to me.