In some apps this feature never appears; while in others its behavior is inconsistent.
In the case of Safari, this option appears to never be available when entering text in to textfields on web pages.
Bug First Encountered:
How often the bug occurs:
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Use an app like Mail to compose your text, then use the Misspelled Words o option to check for errors. Then, select all, copy to clipboard, and paste the text where it is needed. This feature is consistently not available in Notes.