forwarded 1016698 https://gitlab.com/postmarketOS/osk-sdl/-/issues/148
This seems to be https://gitlab.com/postmarketOS/osk-sdl/-/issues/148 upstream. Upstream's current direction is to move to unl0kr instead of osk-sdl, so it's somewhat unlikely that that this will be fixed unless someone experiencing the issue tracks the bug down.
I'd recommend testing whether unl0kr solves the issue if you're on Sid/Unstable.
