Bug in Monster Builder (HP resets to -99999...)

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So this is the Nth time this happens on this monster. Only on a customized Solo.

After I leave and come back after a while, the HP goes from 100 to -999999... and shows up on the summary as -1,96E+7.

If I change it and save it, it's still screwy when I return to the Monster List.
If I change it and save it as a  copy, it'll stay correct only until I log out and come back.

What's going on here?