[BEH]

The leak is probaly from porosity in the weld.
This is a weld defect.
I don't understand why BDR isn't repairing it under warranty.
If there is a history of weld problems, then all tanks should be pressure tested at the factory, either BDR or the supplier of the tanks.
Sealers I would think is a temporary fix or something you would use in a old junker, not a new vehicle.