From what it was in 1.0, assuming, even if it isn't the case because of a bug, it should be a flat amount of xp gained across all classes.

So for use of simple numbers, I. Have racked up 100k xp in bonus xp.
Use 50k xp on blm level 45
Switch to level 4 archer, you should still have 50k to use.

Which is why if you are on a low level class and see xp bar boost all the way to the next level, switch to a high end level 49 class you might not see a boost because its so small in comparison.

But if the bug is causing it to vanish from going from your level 45 class to level 5 class then it needs to be looked into. Any patterns anyone notice when switching classes makes it vanish?