Happens in both 20.05 and 19.11: In the OPAC, when using MARC view for a record with an online resource link, the complete HTML for the link is displayed for the URI. That is, if the URI is: http://some.valid.link, the MARC preview shows: <a href="http://some.valid.link">http://some.valid.link</a>
How was the link catalogued in this case?
I can confirm this in master too. Testing with a record that has 856u populated with a regular link, the MARC view shows HTML. An example from our catalog, currently at v19.11.08: https://search.myacpl.org/cgi-bin/koha/opac-MARCdetail.pl?biblionumber=239528
opac-MARCdetail.pl checks whether a subfield should be a link, and outputs HTML if it is. The template assumes that the subfield value *doesn't* contain HTML and uses the "html" filter on the variable. The template needs a check on whether the subfield is a link in order to selectively use the "$raw" filter.