Even on login the XP gained number is super high before I have even done anything that would give me XP. This occurs on all my characters that aren't max level.

No lua errors are generated while the number is this high.

One thing to note, the gained XP number does go up when I gain experience, it just seems to not reset when I log off or reload my UI, so I'm guessing the number is the results of multiple levels worth of XP gain.

Do note, I am almost max level on all my characters, so I won't be able to confirm if the bug is fixed for much longer.

One thing to note, the gained XP number does go up when I gain experience, it just seems to not reset when I log off or reload my UI, so I'm guessing the number is the results of multiple levels worth of XP gain.

This is actually a good thing to note, will help me narrow down what this might be about.
I've got a few ideas, but will have to check it out to be sure.

Do note, I am almost max level on all my characters, so I won't be able to confirm if the bug is fixed for much longer.
I, unfortunately, can't promise that it'll be resolved super fast as I have some RL to contend with in the coming days.
However I do have a few characters still myself to level and given the previous thing you pointed out I think I should be able to at least fix this even if I wouldn't be able to reproduce this.

Thanks again for the detailed report already, this helps out a ton in narrowing these things down.
It'll probably help me be able to quickly tackle this issue when my RL clears up (this shouldn't be too long).