Okay, I'm now using Openboard and got both working - I believe the issue is related to word suggestions. I've disabled those and it's working now.
Discussion
That makes sense. Did you turn off autocorrect as well?
Should've been clearer, all suggestions and autocorrect.
However Openboard seems to have a dependency between next word suggestions and corrections. Auto-corrections also don't seem to work with suggestions turned off from my testing.
I disabled suggestions and it seems to have fixed it. I left auto-correct on and haven't had any issues so far.