In Lightroom Classic (v 11.1) there is a bug if the mouse / pointer size has been changed in System Preferences > Accessibility. The brush size when using masks shows correctly when the pointer is over the right hand side panel. When using the brush over the image part of the window it shows much larger (depending on the accessibility setting) but it's effect on the mask is that of the smaller size. If the mask overlay is not turned on, it is not apparent what is happening - it looks like the brush is simply not working.
I used the accessibility setting as I use a very large, high-res screen and the pointer is so tiny it hard to see otherwise. I'm using a 2021 Macbook Pro M1-Max, 32GB, MacOs 12.1. The problem is persistent. The problem appears if I use and screen, including only the inbuilt screen.