Beta causing battery to drain whilst charging

Have you tried plugging your phone in with different cords, power adapters, outlets, electrical circuits, etc.? It would be helpful to verify that the issue is in fact the phone or the software and not the external charging hardware.

Betas are notorious for lack of proper power management, so yes, I'd expect that's the problem.

I'd run the battery all the way down, let it sit for a few hours, then put it on charge without powering it on and let it top off, then power it uo and go about my business.

Be sure it's plugged directly into the computer while charging it back up, not thru a charge brick, external drive port etc.

Good luck.