It seems that you have to be in survival to use the xp storage. It doesn't let you do any xp transfer actions and will not allow you to enable or disable vacuum mode in creative mode.

Allowing creative interaction can be useful for testing purposes when designing farms

Good point, I change this with the next update. Maybe when remove xp while creative it clears it in one go?

yes this is intended behavior, maybe i will change this at some point.
whats the benefit to allow creative insertions?

fixed with the newest version