I think @MimeticMouton is on the money here, certainly for DOCX and anything pasted from a website. In my experience, both of those litter no-break spaces throughout a document. Paste-and-Match-Style doesn’t get rid of them as they are regular Unicode text code points, not control characters (a.k.a. Gremlins).
There’s another thread, where the OP was having trouble with similar recalcitrant non-printing characters on importing from Word. It turned out the document was peppered with no-break spaces and tabs.
Mark