Not sure what I did, and I can't get it back. (On a PC)
Press q for quick mask - ok
B - ok
paint - ok
exit quickmask - ok
cntl-shift - I - ok
Cntl J - ok
v for move is where I have a problem. It no longer moves the part that I painted with the brush, it seems to move a whole copy of the image and leaves a whole where the brush was.
I have used this technique hundreds of time, but now I have some setting messed up.
Select move > click the little arrow to the right of the tool icon >when the box opens click the little arrow on the right side. A dialogue box will open with a multitude of options. Locate reset tool and see if that helps. If not try a couple of the other reset options.