I recently made the swtich from DDG from Searx simply bevause right-clicking on a search result to copy the url resulted in a referrer link to be copied rather than the link of the result destination.
I think enabling DoNotTrack header or disabling JavaScript prevents this behavior: i cannot reproduce on Tor Browser. But you are correct this is a worrying development.
But this same behaviour cannot be observed on Google/Searx on the same browser. It also even isn't observable with DDG on a FF nightly build on the same system and a FF stable build on a separate one.
Edit: url formatting
EDIT2: this behaviour seems to be limited to my particular setup and even a clean LibreWolf profile seems not to suffer from this issue. I apologise for the misunderstanding.