I bought Photo (version 1.6.7) from the Apple App Store yesterday and viewed the welcome tutorial.
I'm using a MacBook Pro, mid-2010. 17" 2.66 GHz 8gb ram under MacOS 10.13.6.
I have tried to use the Undo Brush but note a couple of things, 1, that the version of Photo I have doesn't appear to work like that in the tutorial and 2, I do what appears to be the right thing but the undo function just does not work. I also note that in the tutorial there is mention of selecting an undo layer in the Adjustments menu but I can't find such an option so selected the tool directly as I'm still in an adjustment layer. Obviously I'm doing something wrong as all the other adjustments I've tried worked fine.
Anyone got any ideas?