A family of Microsoft word processing software products for creating web, email, and print documents.
There are several possible reasons, most of which stem from the content being copied. It's difficult to discern the specific cause without seeing an actual document which illustrates the issue. If you save a copy to OneDrive & provide a share link in a reply here I'm sure someone will be able to assist. See this Support article if you need guidance: Share OneDrive files and folders
At the very least, provide a screen shot of the document window with the non-printing characters displayed. That may identify the cause.
Also, please include the specific Version Numbers of both macOS & Office.