It was happening all the time, pressing M does not produce anything. However now it seem the opposite is happening, I adjust the exposure and then press Show Original and it does not respond and just stays on the edited image. I just now checked it again and it the original flashes but now it went directly back to the edited version but the text unedited image is displayed when pressing Show Original which does not work at all.
I just tried clicking on another image and then back to the test image and it started working correctly. If I stay on the test image the issue returns, the edited image stays displayed when clicking on Show Original and will not show the original at all.