TinyMCE spell checker overrides browser spell checker, even when disabled or uninstalled

With the new framework for controlling TinyMCE plugins in the same way as other Moodle plugins, I was hoping that it would (at last) be possible to disable the TinyMCE spell checker. In my testing, however, I found that disabling or uninstalling the TinyMCE spell checker does not have the effect of restoring the native spell checker in the browser.

Replication steps

1. Log in as admin
2. Navigate to Settings > Site admin > Plugins > Text editors > TinyMCE HTML Editor > General settings
3. Disable the Spell checker plugin
4. Save settings
5. Purge caches
6. Navigate to a course
7. Create a Page resource
8. In the HTML editor, enter some misspelt words

Expected result: Misspelled words should be highlighted by the browser's spell checker

Actual result: Misspelled words are not highlighted