It is a stinker.
Remember that you can switch to other companion when you are engaged in conversation.
Sure this is possible, but then we're really into micro managing something you might not even notice until it's too late.
Any (game) design (in my view) should be targeting the best way "it should behave", not focus on the possible workarounds.
And since this happened to me several times during my playthrough, forcing me to replay a looong combat sequence, it was one of my bigger causes of frustration. This, and the lag/delay in character selection when clicking portraits accidently sending the wrong person on a run bugged the hell out of me xD