Can you please explain in a little more detail what 'not working' means in your case? It could mean many things and I have no problems with Lightroom Classic 9.1 and Catalina 15.2 with the adjustment brush or with photomerge.
What does 'doesn't work anymore' mean, please? Can't you select the brush? Can you brush but don't you see anything happening? Have you made sure that the Flow and the Density settings are not (close to) zero?