Medallion drop rate that gives 6 Energized Crystal slots seems bugged

I'm starting to become convinced that the medallion that gives you 6 extra crystal slots (to save up crystals for more temple runs) is bugged and simply stops dropping after some point.

I made a 2nd character and focused on getting to 60 slots before anything else there, and I was able to do it without any issues, but my original character has been stuck at 54 slots. At this point, I've killed the Architect and Atziri many, many times, and this medallion simply does not drop anymore. I can't finish capping this out to 60 like intended.

I have a feeling that after some sort of condition you meet, other than capping it at 60, this medallion is simply getting replaced by some other medallion in the drop pool, because it just doesn't show up for me anymore.