N4CHEM, 1 month ago

Found the solution to this some days ago: you need to change your System Spell checker to Heliboard.

Now the developer added it to the FAQ too:

Spell checker is not checking all languages in multilingual typing: Make sure you actually enabled HeliBoard spell checker. Usually it can be found in System Settings -> System -> Languages -> Advanced -> Spell Checker, but this may depend on Android version.