I don’t know how it works on macOS, but on Windows copying to the clipboard can provide content in multiple formats (like multiple MIME types) at the same time, for example simultaneously as rich text and as plain text. The receiving application then decides which format it understands and selects for pasting. If both sides understand rich text, this results in rich text being transferred. (It basically works like HTTP content negotiation.) You have to artificially add a man-in-the-middle to force a different format.