Gboard's "Spell check" does not work: why and how to fix?

I have an Android 10-powered smartphone with Gboard (v12.9.20) installed. In Gboard, the "Spell check" is enabled. The problem is that the spell check does not work: if I type in the Gmail app "hellowx" (for example), it isn't marked with red underlines. What I've tried so far: - clear Gboard cache - clear Gboard storage - force stop Gboard - restart the smartphone No effect: if I type in the Gmail app "hellowx" (for example), it isn't marked with red underlines. Any ideas why the spell check doesn't work? How to fix it? --- UPD: It works in Gmail in "Subject" input field. It does not work in Gmail in "Compose email" field: Very unexpected. Is it a bug in Gmail? --- UPD2. In Gmail desktop web version I selected "Spelling suggestions off" (UPD3: AND clicked the "Save" button at the bottom): The spelling keeps working in Gmail Android app in "Subject" input field, but keeps not working in Gmail Android app in "Compose email" field: The Gmail Android app was re-opened. Any ideas how to proceed?