The "Remove results from this site" feature not just isn't working, but is actually highlighting the result as the top result - when it wasn't before asking to exclude it.
I did a search "mri gallstones" and then selected the ellipsis menu (three vertical dots) on the unwanted result, then clicked "Remove results from this site"
Screenshot after performing this action:

This is on desktop Firefox 140.0.2 under Linux (Ubuntu 24.04.2), with the uBlock Origin plugin active.
I expected the unwanted website (healthline) to be excluded from the results.