Hey @SLKRadiologie. Welcome to the forum and thanks for the question. 
Sounds like read-only is causing some gremlins.
Do you happen to have a Github account?
If you do, can you submit an issue to the:
After you submit the issue, the QA team can verify and a dev can look into it.
I suspect it would just require an enhancement to let a user in “read-only mode” turn on/off spellchecking.
You’re welcome.