I think it could be the ROPS making it look out of plumb. But these bxx20 backhoes mount a little different than a
BX25. The BX subframe was a part of the tractor frame. The b has a subframe that mounts to the main tractor frame(your already aware of this), and it has a little more play to it than a BX. Once you start using the BH, the pins will wear, and the pins on the subframe will start to wear also.
My fel is out of plumb. When I park the
b2620 in the garage, the left side of the bucket touches first. If I could finish grade, with the fel, this might
be a concern. But I don't have that ability. I parked the
BX25 in the same spot, and the bucket was always level. When I asked the mechanic at the dealer, about the unlevel fel, he said that was not uncommon.

Enjoy your new 2620. They are a step up from the BX.