Another occurence of this peticular issue. It would be nice if sombody from Wizards with knowledge could look into this issue and give some feedback on what exactely is happening. This is the forth time that I have received gems instead of Retro Artifact mythic cards despite not having a complete playset of the later. This is pretty annoying. The first report I filed for this issue has been almost 3 weeks ago, still no official reply.

Same occurrence showed up again. Gem instead of mythic retro artifact. Still no complete play set of those.

This happened to me as well just now, received gems instead of rare retro artifact when not rare complete. I am assuming it was due to me being rare complete for the regular BRO set and I received gems as well for the BRO rare in the same pack.

If that was the case then perhaps you opened one of the banned Retro artifacts (Mishra's Bauble or Phyrexian Revoker)

In this case these 5 rares shouldn't be included in the Boosters at all. However these are all from the Brothers War set and not from the Retro Artifacts set while the gem card showed the Retro Artifacts icon at its bottom. Also there was a standard Brothers War rare in this booster as well, so I can only assume the gem has been awarded for a retro artefact card ... and i dont have a playset of those. This has me thinking there is something not properly working with the collection evaluation .. or the booster contents in case your "5 rare" assumption is correct.

There are 5 rares you receive in the Mastery Pass unless you craft them (which you shouldn't)

The Brother's War Booster with Retro Artefacts: gems instead of rare retro artifact card despite not having a complete playset