I have probably seen the same problem on my Debian/Linux not long ago.

I was unable to paint with the mouse when the tablet was plugged in. Sadly I don't know what triggered this (I suspected xorg update), and it seems to be gone right now.

Yes, there are drawbacks. You lose tablet pressure sensitivity with "Mode for input devices" = Disabled. The tests in the previous comment were done without the tablet.
In summary, with "Mode for input devices" = Disabled; menus close fine when clicking on canvas with mouse, and mouse can draw on canvas. With "Mode for input devices" = screen; menus do not close with regular mouse when clicking on canvas and cannot paint on canvas with mouse, but tablet works fine for these functions. Thanks.

In the pressure settings, if the "Mode for input devices" setting is set to screen, then the drop-down menus will not disappear when clicked in the canvas area with the mouse. They will however, close when clicked with the pen on the tablet. Also, this setting will prevent the mouse from being able to paint on the canvas. When this setting is set to disabled, then all these actions with the mouse work correctly. I checked this in Gimp and it does not have the same problem. I also noticed in the terminal window I get the message "Ignoring "cursor" (probably wacom mouse device)". Maybe this has something to do with it? Let me know what you think.