I have turned off the automatic language detection on both documents so that is not an issue.

And I copy and paste without formatting so it should clear all the issues, but that is when the spelling errors show up.

Therefore there shouldn't be any formatting, style or font issues.

I have checked the language settings on every word and random characters throughout the original document, as well as in the new document AFTER I have pasted without formatting and the language throughout is US English

The document was written by someone in USA so US English should be the default language for the entire operating system and there should be no Cyrillic/Latin character issues but I'll see if that doesn't have any effect.