Thanks for reporting this bug, hitme101!
The root bug is not Champion/Necropolis, it happened one turn before, when you started to pay back your debt from Mountain Pass, with Sauna in play.
I assume you clicked the "6 Debt" button (with one of Copper, Silver and Gold in your hand), then decided that you want to trash the Copper when the autoplay played the Silver and the Sauna effect kicked in. If everything went correct, that should result in a trashed Copper and only 5 debt repaid.
However, it's a known bug that in that case the game will incorrectly think that the originally selected 6 debt were repaid, but it also somehow 'knows' about the missing $1 payment. In other such cases, even when the debt has been fully repaid, the game will always offer you cards to buy for one less than what you have available (e.g. if you have generated $6, it will only allow you to buy cards costing up to $5).
I'm not sure why it behaved differently in your case, maybe because the $6 you intended to repay would not fully repay the debt. Nevertheless, when I replayed that last round and explicitely choose to repay only $5 (and trashing the Copper with Sauna), the next turn (where it stopped working) worked as it should.