How can I get Apple's spellcheck to work consistently?

Something must be wrong with my spellcheck. It can't seem to spell Apple's operating system, even with its spelled correctly. It often misses very basic words. Seems to happen on all of my computers.

I've included a pic where it can't seem to spell Big Sur but can get a more obscure word, like workaholic.

This is currently happening in Catalina, but has been an issue going back to Mojave, at least. I'm thinking that Apple is constantly scanning my Photos library (as evident by the ever present "photoanalysisd" process) and even auto-mounts network volumes to get at Photos libraries. SO why not use that kind of power for the customer? It seems Apple should be leveraging the internet for spell checking vs. installing a huge dictionary on every machine.

Is there a way to use a different dictionary for spellcheck than Apple's but still have it integrated with the OS?