I'm not sure whether this is a 'bug' per se...
I've been thinking a bit about this, and I think zylophone is actually the main enabler of broken things with any card that doubles things.

See my post at:

https://steamhost.cn/steamcommunity_com/workshop/filedetails/discussion/1610056683/3203652426717107638/

The gist of this is that: Zylophone can break the game if you can get over 30 energy worth of X cost played (not that hard with chemical X and echo form effects), minus 1 energy for every power-of-two of the doubled resource present. For the basic "catalyst+" card even less is required as each catalyst triples.

N.l. ceil(3log(2^31-1)) energy. ( This ends up being 20 energy, or 10 energy with an echo potion. Even easier to get.).

It turns out most of the effects and things are stored as integers. Which cap at 2147483647. Unfortunately the game doesn't check for integer overflow when it does math, but it simply does UndefinedBehaviour instead. (Which, on x86, ends up being "ignore the higher order bits, and interpret the 32nd bit as a sign bit")

I wonder if you could trigger this in the base game with nightmare/catalyst against the heart. It seems possible, if a little contrived; you would have to go out of your way to do it.

Not sure how to exactly fix it, this seems like a tricky problem. Maybe the base game spire devs should enable variable promotion for big numbers, or check for overflow?