There’s so many basic words that it just doesn’t recognise. I’ve got the spellchecker language set to Australian English. Apparently “prized” isn’t a word. “Beseeched” isn’t, though “beseech” is. “Draconic” isn’t a word.
There’s so many examples of words, which are words in Australian English (which is really just UK English).
If you don’t appreciate the Scrivener spellchecker, you might turn it off entirely and install an external spelling and grammar tool that integrates with Scrivener.
See this thread for how to replace the default dictionary with a more robust one. (The thread is in regards to the v3 beta, but still applies to the production version of v3.)