However, it's any GET vars that seem to be copied. Doing the same thing on https://www.w3schools.com/?abcdef=facebook has the same effect. It only seems to happen on W3Schools too. I suspect it's the site trying to be do something rather than anything nefarious by Chrome.
Load https://www.w3schools.com/?utm_source=facebook in a new tab.
Highlight "https://www.w3schools.com" in the URL in the address bar.
Copy.
Paste in to Notepad and get "https://www.w3schools.com/?utm_source=facebook".
However, it's any GET vars that seem to be copied. Doing the same thing on https://www.w3schools.com/?abcdef=facebook has the same effect. It only seems to happen on W3Schools too. I suspect it's the site trying to be do something rather than anything nefarious by Chrome.