It should allow me to choose the on-buy order (haggler or Mint) but the interface originally says "you may buy cards" even though no card is clickable (I have buys left). I don't know if this would happen without Watchtower in hand, but I think I've seen that work properly before.
Screenshot attached.
Thanks for the report, pablominion!
Buying Mint under Haggler/Swamp Hag is a known issue, it even came up yesterday in another thread. The Watchtower is not relevant in this case.
Buying Mint was "always done first" in the past, which made this scenario work, but it wasn't correct. The change however introduced this bug, unfortunately.