Afaik this is just a display bug that happens when you transmute stacks. reenter the potion screen and everything is back to normal.
Thanks for reporting this @ProteinPig. What a shitty bug is that :-( Terribly sorry that this bug ruined your bonus round. To make it easier to actually find this bug - do you remember some additional details from when this happened? Like how many other potion cards you where holding, how many mead bottles you transmuted? Thanks for your help!
It's about 34 Great Potion of Crit 10-20 Great Potion of Strength 40-60 Mead Bottle 0-10 Potion of Water and several other potions But, I don't think it's worth to pay too much attention on it. I played another hard game today, and nothing happend. BTW, I survived 22009 with Kvote, and leveled 2 up.