i have found a build that keeps filling the tanks with XP without draining your your XP bar. as you can see below, it requires quite a bit of XP for a setup, but once you have it, you can fill them up, making the tanks bigger and bigger(i built a house out of XP tanks on another server with this trick).
it seemed like you almost fixed it in version 1.2.5 by it draining all but 2 bars of your XP(this is something that's been in this mod for several versions).

Fixed in 1.2.8

I can confirm that I have had this issue aswell. I was going to take pictures and such but you beat me too it!

Also works in this setup. The last tank being the one that bugs