I think this is a result of auto-formatting on Airtable’s part. If you enter www.someurl.com into a single text field, it’ll become a link as well. Below is my attempt at solving, but I wasn’t able to either. This might be worth contacting Airtable about.
""&{URL}
would have been my first thought too. I was able to get proper results when concatenating a string with a value (such as “1”). But after running a substitution (even in a separate column) to remove that added character, it reverts to URL form.