When I tab out of Canvas only mode it goes directly into full screen mode even if I wasn't originally in full screen mode. If I tab back into canvas only mode and then tab out again, then I'll be in the regular view mode.
This is fixed in master, but in 3.1.1, full screen seems still completely broken.
In krita 3.1.2, when you are in maximized window mode, tabbing into - show canvas only - is still buggy.
I cannot reproduce this anymore...