Why does some of my heroes have negative xp crystal costs?

I decided to fire up my Marvel Puzzle Quest over the holidays, after not having played it for a while. After the game updated (on my PC) and I got into the game, I noticed that some of my heroes now have negative XP crystal costs to level up. Why is that? Other of the heroes are ok. New heroes recruited do not have this issue either. So what is causing this issue?
Best Answer
I cannot find a root cause for you, but here's a post from the D3 Go! forums:
One of the moderators suggested this:
Try playing a Prologue fight with your bugged character(s). Like mentioned above, a little while ago, there was a change in ISO costs, and a while before that there was a change that caused a similar issue - in the latter case playing a game with the character fixed it. I'm not sure about here, but it's worth a shot.
But, the poster reported that, in this case, it didn't fix the issue. Another moderator suggested opening a support ticket with this link:
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