If you have a ducat in your hand, the game won't let you pay debt (and by extension buy a card) without playing the ducat.
I hit this scenario when i wanted to trash a ducat with a bat after buying a card. It was impossible.
You are right!
That might have to do something with autoplay optimizations: If you have 1 Ducat and 3 Coppers in hand and buy a Silver, with autoply the game plays all of those 4 treasures, giving you the $3 for the Silver and the 1 Coffer for later. Maybe that crept in here as well.
Thanks for the report, and don't panic!
Thanks.
For what its worth, I don't enable any of the autoplay optimizations, and I didn't hit autoplay all treasures.
This is really odd.
Even moving the slider to some amount of Debt that should be covered by what coins you already have triggers the Ducat play.