How are you inverting the mask? Using a shortcut or selecting Invert Mask?
I'm trying to invert using the menu option from the Develop screen.
I just tried the process again. It still happens from the drop down menu, but not if I choose "invert" from the sidebar controls, so I suppose that is the workaround. Actually easier anyway. Thanks!
The shortcut ' (single quote) also toggles it.
Restart Lightroom and see if that helps.
Got it. Thanks!