With https://github.com/openSUSE/instsource-susedata/commit/9b4a88af4245202d4f8f1b8fa50e5ec63c376c5c the AppStream metadata should now be correctly registered in repomd.xml, as produced by the product builder. Things produced by OBS are still inherently broken, but that's beyond the scope of this very bug report. Will keep the bug open until I could confirm/validate the result (either in SLE/Leap or TW - they should all happily be fixed now)