For those playing on a gamepad w/ movement mapped to the D-pad, you may have noticed that diagonal movement doesn't work. The reason why is because if you look towards the controller config menu, you'll notice that the buttons on the D-pad are recognized as POV switches rather than separate buttons. As of yet, I know of no way to get 8-directional movement confined to the D-pad. However, there are alternatives:

- Map movement to the L-analog stick.
-Map either left/right or up/down to two separate buttons such as the triggers while leaving the other two directions on the d-pad. Ex: R2+left on the d-pad will make you go northwest and L2+right on the d-pad will make you go southeast.

I'm still experimenting myself on trying to come up with the best combination, but this seems to be the best I got right now.

Turns out there is a solution to this.
https://www.gog.com/forum/metal_gear_series/dualshock_4_dpad_metal_gear_solid

That's fair. Until then, reassigning to the L-stick or separate buttons seems to be the best option.