I don't have the impression that the Proton Mail Android app development is slow, nor that it is a disaster. I've been on the beta channel on the Play Stores for months, and it's been slowly improving, with regular releases. They have a web app which is good on mobile, but lacking a bit in convenience because of the small screen and it's not really optimized for it. I'd like the Android app to have more features, but I think it's currently allright.
Back on topic: Openboard seems to me to be largely unmaintained. I see that it hasn't received a commit for the past year, and the latest release was two years ago. Might it be better to switch to a keyboard app that is more actively maintained? I think it's really the only option you have if you want to avoid this bug, short of discontinuing to use all the apps where it manifests. Florisboard is often brought up as an open-source alternative. I just tested the latest Florisboard alpha release with Proton Mail, and I didn't experience any issues.