So, regardless of Big Picture settings, in-game settings, and if I use X360 or PS4 pad, it's the same result.

HOWEVER, I may have misinterpreted the issue. It may not be that the shoulder buttons aren't working, but rather I somehow can't figure out how to make the hotbars setup for them properly?

So, L1/LB is defaulted to auto-run. And that works fine on both PS4 and X360 pads. But L2, R1, R2, etc are all bound to hotbar-oriented buttons. So I'm starting to think it's more of a UI/hotbar issue than a controller issue? Do I somehow need to have the UI/hotbars look like they do on console?