If you have enabled the auto-pause option "Character's target destroyed", you can gain control over your party during the dream sequence where "Imoen" gives you the ability to change into the Slayer at will. The auto-pause triggers when the Slayer avatar is about to kill the Imoen apparition.
This issue was already present in the original BG2 (Link).
Do you have a saved game where the next rest will trigger the dream sequence? That would be important for testing a fix, and if you have one already it will save someone some valuable time.
Comments
@Troodon80 this issue needs a number and to be added to @Illydth's thread.