This is due to a special configuration done on svwiki where the interwiki prefix is src instead of s.
This special case is handled in WikimediaMaintenance DumpInterwiki with the following code :
/**
* Special case prefix rewrites, for the benefit of Swedish which uses s:t
* as an abbreviation for saint
*/
protected static $prefixRewrites = [
'svwiki' => [ 's' => 'src' ],
];Sadly it's hard to reuse such information because the InterwikiCache generated is not suited for the kind of lookup we do in CirrusSearch.
Externalizing the config from DumpInterwiki to wmf-config is also not trivial (wmf-config generates a config per wiki but DumpInterwiki is designed to work for all wikis by inspecting database files and such)
The sole solution that does not involve a giant refactor of how interwiki data is exposed from core is to duplicate this information as a cirrus config var.