When mirrored or moved at all it empties the sketch it was on and makes a new one, even on the same plane. Unless I missed something this seems odd and makes things hard. I can’t select a body after a mirror due to separate sketch planes.
Edit:
I am also unable to select those sketch planes once they go to 0 sketches. I can still slide to left and erase.
Version 5.470.0.5817 4944246c