Problem using brush in Photoshop 2025

Hi everyone,

I'm quite expert in Photoshop but having a weird issue for the first time since a few days...

FYI: I'm working on a MAC, on the last version of Photoshop.

Everytime I use the brush on the mask, first problem is it only works with some trasparency instead of plain colour(I already checked the opacity of the layer and on the top bar menu), so for example when I use black to erase the part of interest it's not erasing fully due to the transparency. The second issue, still while using the brush in the mask,  is that when I click every single click it's switching from white to black to white and so on. In the left instrument menu the colors ar not switching (black stays as foreground and white as background). Both these problems happen also when I use the brush on a normal layer (and not on the mask). I'm sorry I hope I was clear enough, I am attaching a video to help you better understand what I mean. Any help will be fully appreciated, thank you. 

Lisa