I found a way to fix the issue: rotate the button back into position (turn it clockwise) using an X-acto knife. Then place a small square of duct tape over the button to hold it in place. The square must be large enough to stick to the button and some of the metal around it. You can still use the button through the tape, and the tape prevents it rotating back into the bad position.
My X-T10 is out of warranty, and the thumb rest has fallen off, along with the rubber grip material on half of the camera, and this button failure. I haven't abused this camera - they are simply not built to a very high standard, I'm afraid. It still takes wonderful pictures, but I can't say I would recommend anyone buy one of these.
