The basic notion is that the rules of multi-classing in D&D 5e state that a character who chooses to multi-class must meet a stat requirement of both classes.
Now, what happens if and when the character is True Polymorphed into another creature that may have mental stats that don't meet the multi-class requirement? Especially if they no longer meet all stat requirements.
Can they still gain experience, and how do they decide where to apply it?
Typcially all characters are allowed one class even if it doesn't meet the stat requirement, but nothing covers what happens in the above eventuality.