Turns out that the problem is a syntax error. This line:
<str name="langid.fl">focus, expertise, platforms, partners, participation, additional</str>
must be
<str name="langid.fl">focus,expertise,platforms,partners,participation,additional</str>
The docs state that the field list should be comma or space separated values. Evidently, comma and space screws things up (though it works fine in other Solr contexts like fl
in a requestHandler which langid.fl is supposedly modelled on). I tried the space-separated syntax as well, but it did not fix my issue.
I hope this helps someone.