Upgrade slots keep emptying in BTTG

I'm having issues keeping players in the upgrade player slots in Back to the Gridiron. If I put an Uncommon player in a Rare upgrade slot or Rare in an Epic slot they won't stay in there if I back out of the slot. Epic players slotted to upgrade to Iconic do stay in the slots so far. This doesn't happen for any other trades, just for BTTG.