Every time I try to use the brush tool on a mask, the brush tool changes to the Eraser tool every time I am in the middle of using the brush tool. I change back to the brush, start to use it and after a few moments it changes to the eraser tool again. I have tried removing the eraser tool from the toolbar but it still, miraculously, returns when using the brush. I am using Mac Big Sur and am on the latest Affinity update 1.93. This is getting very frustrating. I have tried closing down and reopening Affinity, and rebooting my Mac, still to no avail