In both intranet and OPAC XSLT, if a record contains a (now MARC-standard) 490 instead of the (now obsolete) 440, the series search link in the biblio detail page uses (the undefined-in-MARC) $t to create the link to search in serials, rather than the $a, which is what it is correctly displaying.
Created attachment 3433 [details] [review] patch
Pushed please test