It wasn't spell checker, it was the auto replace that was causing the problems.

Jul 17, 2018
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I kept having problems, when texting, with words that had been typed correctly being replaced by the auto replace tool. Even after having corrected it twice to the word I wanted---- Donair---it kept wanting to replace it with Donati. After removing the Auto replace for English, under applications Settings, Language, I was able to type Donair without getting a popup on the screen and spell checker still worked. Some upgrades cause more problems than they resolve.