I was on the Wikipedia article for Terry Carr and clicked on the link to ISFDB that's in the External Links section. This went to http://www.isfdb.org/cgi-bin/ea.cgi?Terry_Carr which ISFDB reports as "Author not found: Terry_Carr". Wikipedia's help page for their {{isfdb name}} template at https://en.wikipedia.org/wiki/Template:Isfdb_name includes an example with http://www.isfdb.org/cgi-bin/ea.cgi?Isaac_Asimov That link is also broken leading me to believe something changed on the ISFDB side that broke the handling of _ (underscore).
Anonymous
This was indeed an unintended side effect of the fix implemented to address Bug 774 "URLs with non-English characters fail".
Fixed in common/isfdb.py, installed in SVN 660 on 2021-06-23. Closing the Bug report.