I’d submit a bug report but I’m not entirely sure this can be reproduced consistently. I’m also not sure whether the bug is with Kwin, Plasmashell, or just the Settings Manager…
It seems like a new feature was added to make controllers mimic keyboards. This could be useful sometimes, but for some reason it’s enabled by default instead of being an opt-in, which seems very questionable.
What’s worse, it (or at least the ability to turn it on and off via the settings module) seems very buggy. There’s a toggle switch but it doesn’t work. Earlier I couldn’t turn it off, which made Celeste unplayable.
I just ran an update and logged out/back in, and now it’s still listed as on, but keyboard inputs are no longer generated. Either way, if I toggle the switch off then close Settings and re-open it it will be shown as on again. And either way, toggling it has no immediate effect either (fortunately now it at least seems to be stuck off instead of on).
EDIT: I was mistaken, it’s still stuck on 
I can’t turn it off and it’s unusable for gaming. I guess this is what I signed up for switching to the unstable branch