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.