I've seen it again twice now, with different cards in the discard pile. So it's definitely not a Sceptre thing.
I think it might be related to having Fortune in the kingdom.
I noticed it when spectating Dan Brooks' recent match. Game 31084629 on Turn 18, but I haven't been able to recreate it with Rattington.
When playing around with that seed, I also observed that Fortune sometimes loses the debt symbol in the opponent's discard pile. Part of the same issue?