OS: Windows 10 Browser: Google Chrome Pages: https://subeta.net/games/battle/exp.php / https://subeta.net/games/battle/exp.php?pet=6001720 Pet: https://subeta.net/petinfo.php?petid=6001720
Walkthrough: I had been progressing through the battle quests with my pet Liabeau, immediately spending the EXP reward after every level with no issue. Upon completing the level 5 battle quest and obtaining the 500 EXP reward, I once again went to the Spend EXP page where I could see that said amount had been awarded, and utilized 450 EXP to increase Liabeau's health by 3 points. The health increase was successful, but it left my pet with a negative EXP amount, as if the 500 EXP hadn't been properly credited/was revoked while still allowing me to spend this "phantom" EXP I didn't actually have. Edit: The issue might be multiclick related, as my current mouse will sporadically output two clicks when I've only pressed LMB once. I have refreshed both pages and checked on a different browser and the value is still displayed as negative. Earning further EXP from normal battles reduces the negative amount as expected. (ie earning 14 EXP would make me go from -324 to -310) I could not beat the level 6 battle quest to test if further EXP rewards from those were awarded as normal.

[Tree=Joii]
Id go to the coliseum and defeat an easy opponen, outside of battle quests, to check the EP award. Then you might have to file a ticket.
It happens when the site lags / multi clicks from time to time. It's an issue that unfortunately a programmer would need to fix and could take some time. You can make it even itself out just by winning a couple battles as Turokai mentioned.
Thank you both for your replies! I'm aware I can fix it myself and intend to do so, was just reporting the bug in case it was a new one since I didn't see any threads about it! Sounds like it was the multi click thing, as my mouse has had that problem lately.
[Tree=Joii]