I’m of not much help here as I do not use the spellchecker - but I forget to disable it (when switching keyboards it’s all underlined red). From looking at the settings, the selected system-language gets precedence on the spellcheck, not by selected keyboard-layout: AOSP Spell Check
Did this change from versions? Were you on 0.20-oreo stable and updated to 0.21-q, or did you stay in the same Android version (just updated /e/ version) ?
you could try other keyboards, a user as asking for this: spellcheck by selected keyboard-layout-language: Alternative keyboards for /e/?