Game #2605976
On turn 9 - ceviri, Contraband is played and the opponent names Avanto, however the log shows that Sauna was named. Avanto was unable to be bought, so that part is working correctly.
I would guess that this issue is not unique to the Sauna/Avanto split pile.
I tried to replicate that with Encampment/Plunder and the log was OK this time: I choose Plunder and Plunder was stated in the log as forbidden.
Good news is also that this works mechanically as it should be: If you forbid Plunder and 1 Encampent is on top, you can first buy the Encampment but then can't buy the Plunder. If in the same situation you forbid Encampment, you cannot buy Encampment. If you gain it via Horn of Plenty, you can then buy Plunder.