at time acceleration beyond 10,000x ZBO tanks still boil off hydrogen. I'm not sure what else to add, steps for repetition are to put a ZBO tank out on the launch pad and time accelerate to 100,000x.

Hmm... seems like it might just be that (float-point error). Though I would think that X * 0 would always == 0, but perhaps the floating point implementation isn't as smart as that :)

Even more strange is that there is boiloff, but no cooling cost. Might actually be a bug in the implementation in regards to timewarp (it is not specially handled at all, instead relying on planetarium universal time and calculating a delta between updates).

Will look into this shortly, should have it fixed up for the next release.

The boil off is low enough that I'd expect it to be a floating-point error

Having looked into this; the problem is not that the ZBO stops working, it is that you run out of EC and/or the EC is not updated properly; it could even be that at very large time warp that your vessel does not hold enough EC for a single update.

Going to do a bit more investigation using some RTGs for power generation to see if I can keep the power generation in line with the ec costs.

Indeed, with sufficient power generation and power capacity it all works as expected and ZBO tanks do stop boiloff even at 100,000x timewarp

Closing as a non-issue; feel free to re-open if you can provide further information.