ScreenSaver APIs have no effect if the screen has already started to fade
This bug was reported from our Proton test lab; there's code in Steam that calls org.freedesktop.ScreenSaver.Inhibit, and org.freedesktop.ScreenSaver.SimulateUserActivity when controllers connected to Steam are interacted with. That works fine to avoid screen locking while playing a game.
If you stay idle long enough for the screen lock fade to start kicking in, then interact with your controller (which sends the above APIs), the screen keeps fading and eventually locking/blanking. Only moving the mouse can interrupt this process.