I don't know if piper has been updated since being included with Rocksky's scrobbling api, but it seems that the records for teal.fm are in need of fixing. I've noticed the issue of multiple other clients having no album art referencing teal.fm data, and it seems that's down to the casing in anything with an musicbrainz id (in Rocksky's piping, it is always _Mbid, but other implementations use _MbId.
I am unsure if this is the only issue, but I know this much is the main discernable difference I've noticed.
Thanks for reporting this and for the detailed investigation! 🙏 You're right — the issue was related to the MusicBrainz ID casing (_Mbid vs _Mbid). I've just pushed a fix that normalizes the field, so teal.fm records should now resolve album art correctly. Feel free to test it again and let me know if you still notice any issues.