Spell check flags correctly spelled words as incorrect

This bug has been around since Mojave, and it's still not fixed:

In applications using the system-wide spell check, correctly spelled words are sometimes flagged as incorrect. The spell checker then suggests words that are wrong in this context, such as “they're” instead of “there”, “piece” instead of “peace”, “too” instead of “to”, “its” instead of “it's” etc.

Here's a screenshot:

Note:

• This bug is not app-specific. It affects all programs that use the internal spell check of the OS.
• Activating/deactivating “Check grammar”/“Check Grammar With Spelling” doesn't change this behaviour.
• Setting Spelling from “Automatic by Language” to “British English” or “U.S. English” doesn't solve the problem.

Fun fact: As I typed this text, the spell check even flagged its own menu entry as incorrect: