Mask color picker changes my choice from "color indicates 'affected areas' to 'unaffected areas'".

In ACR (via Bridge) the Mask color picker keeps changing my choice from "color indicates 'affected areas' to 'unaffected areas'" at it's own whim. Is there a keyboard shortcut I'm hitting that is setting this off? It's not in the list of documented shortcuts. A bug, I think.

ACR 12.2.1, Mac OS 10.14.6.