Not sure if thats bug though, but I encountered it during play today:
Ninja Tower is clearing Oni counter on winning the stage - that’s ok.
Not sure if thats intended, but this doesn’t happen on failed stage. I had Gadeirus left with Oni counter on 2. He went down before the counter zeroed, and I have failed the level - and it looks like curse was automatically applied on Gadeirus, knocking him out of usability in tower (he is 2/2 now).
I thinks that this is bug as you have no possibility of cleaning the counter - yet you just failed the stage.
I’ve also seen counter on single fallen heroes, but - as you are still in play - you can clear that off, so it’s not big deal. The first thing seems like a bug for me.