Netti93's follow-up report (single artist at download time, correct only
after retag) reproduces as FIXED on current dev — verified live against
Deezer's API with his literal track ('VERLIEBT IN MICH', FAYAN feat.
Dalton) and his exact config, through the real tag writer onto a real MP3:
TPE1=FAYAN, TIT2 gains '(feat. Dalton)', TXXX:Artists=[FAYAN, Dalton].
His last test (2.5.6 / May-19 dev) predates the fixes that closed it
(d5de724f contributors upgrade hardening, 0769fcd5 collab-tag loss).
These tests pin the full direct-download shape so it can't quietly
regress: Deezer /search payload (one artist) + provider on the candidate
(not the context) -> contributors upgrade fires -> feat_in_title and
artist_separator both honored. Network-free (client mocked with the live
API's verified response shape).