Thanks for the suggestion. I tried all of the recommendations (except turning off auto-correct) but none worked. For example, typing the name "Jack" (even though it's in my contacts) will link to the contact named Jack but still CHANGE it to lowercase "jack." I don't expect it to "correct" my typing "jack" but to stop changing the correctly typed name. It's not even like it's an unusual name or rare spelling!
According to Apple's help, autocorrect should learn this after correcting it multiple times. It's a name I use often, I never use the term "jack" (as in a car's jack, or colloquially). Similarly, there are other words that autocorrect constantly misspells - can't think of them at the moment - that I've corrected in other apps like Notes and Mail, but that doesn't seem to carry over to Messages? Oddly, Messages dictation gets it right, even if I'm not texting Jack, and even when the name is in the middle of a sentence, as in "That's what Jack wants to do."
Going in to Settings every time autocorrect is wrong is frustrating, rather than being able to tap-hold on the word (or option-click in Mac or iPad with external keyboard) and say "Learn Spelling." This makes no sense. In this one case, I will add it as a shortcut, but for other misspelled words, that's tiresome.
So there's no way to Learn Spelling in iOS's Messages, even iOS 17's version? Does Messages use a different dictionary than other iOS/iPadOS/MacOS apps?