It goes up to 100 at the beginning of every round, even if it was 0 before you hit start.

not only EV the mystic also has this problem . but in the mystic case it's a good thing!

My friend used to have the problem where the heat would go up to 100 at the start of the round, but after removing EV2 from his deck to check something on another hero, and putting her back in it, it stopped happening to him.

I reckon EV2 has been fine for him ever since.

It only happens when wearing EV2 Mark 3 costume for me.

it can happen on all costiums and can be 100 or 200 heat 

yes it happend to me after buy lavamancer in my deck!

Another issue that may be related to this  is that between waves heroes don't get healed back up, and harbinger buff (yellow mana crystal) doesn't seem to be removed between waves either. This was probably something that was changed in the "journey begins" update. When fixing the "Fixed an issue where EV2 would have full heat at the start of a Combat Phase." issue.  So perhaps the ev2 heat issue was always there, we just didn't see it, because the stats would be reset between waves and now they aren't anymore.

It doesn't matter what costume I wear for EV2 either. And however much total heat I can take is the amount I start with 100, 150, 175, etc). The only work around I know of right now is to start the wave with another hero & then switch to EV2 after wave starts. This way, she will have however much heat she started with before the wave started. NOt the best way to start a wave, but better than having full heat.

ev2's heat does not reset is considered a hot issue, funny pun; but yeah I've noticed this one too but I'm not sure if trendy meant to make it like that

Fixed in the Halloween Update! :)

I can confirm this isn't fixed.

So it looks like there was a partial fix with EV heat and build phase. 

The fix that made it in was EV's heat going to 100 when starting build phase. 

Will move to hot issues again and thanks for bringing it back to our attention.