Why does the new IOS text underline words that are not spelled incorrectly ?

Like Chosen said, it means it got autocorrected. You can touch it to automatically switch it back to how you originally typed.

quote:

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I like the other way better
When they underline it now I can’t tell if it’s because they autocorrected a word a spelled or if it’s because it’s spelled incorrectly

I really wish I could disable it

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I thought this was the case, but just tested to confirm: auto-corrected words are underlined solid blue; misspelled words are underlined dashed red. Once you know what to look for, should be easy from there.