I could not understand why spell check was so unpredictable when trying different methods of producing the red underline, in conjunction with Text > Spell Check > Check Spelling.
Turns out it’s a mode that is toggled on/off each time Text > Spell Check > Check Spelling is chosen, as opposed to running the process on a layer (which I thought was strange so thankfully it’s not that).
If you go through the left “F” menu (not sure what it’s called), there is Text > Spell check > Check spelling*. Or if through the ‘Quick actions’ search, it can be enabled/disabled – both include an indicator.
But there is no indicator in the Text menu bar. I now know, but this should be unified so no one else struggles with it.
*Note that there’s both title case and mixed case across the instances of these menu items (Spell check, Spell Check, etc).