System Information
Operating system: Win 10
Blender Version
Broken: blender-2.83-7c0e28594840-windows64
Worked: blender-2.80-windows64
Short description of error
The sculpting mask brush hotkey (M) used to work as a toggle on previous versions of blender, which means, if you press "M" it would switch to the mask brush, and once you press "M" again it would switch back to the previous sculpting brush you were using. But that's no longer the case, even though the Toggle property checkbox is enabled in the keymap...
Exact steps for others to reproduce the error
Launch Blender
Go to File/New/Sculpting
Press "M" and it will switch to the mask brush
Press "M" again and nothing will happen. (it should toggle back to the draw brush)