It doesn't appear to be borders and shading. I selected all texts and pressed no color. But it didn't change it. When I select all text and clear all formatting it disappears. I work with a lot of OCR texts and I never saw this before. And very often there are numbers mixed in with letters to form some very strange words, and Word doesn't even underline those in red for some reason. Seems spell check only works with alphabetic symbols and not numeric ones.

Select a misspelled word that doesn't trigger a spelling error in Word. Under Review Tab check language - proofing language. Is there a check or shading in the box for "do not check spelling or grammar"?

Mastering the Spelling Checker