I think this is for the same reason that animation overrides don't work properly. I wrote in another post on the AO: This is a bug in the current system. The Al can decide to make gestures and dance poses by themselves to express themselves. However this probably makes [them stand up in order to play their gesture/ animation. This needs to be fixed or IMHO] LL should get rid of it completely. Or at least give an option to the user to switch it off. There has probably gone quite some work to do this, but it doesn't work. Sometimes it is better to let go of a feature that brings more problems than benefits.

This is likely the same bug as https://feedback.secondlife.com/character-designer/p/bot-standing-immediately-after-siton-is-offered

Yes this is the same issue I get too.