In P+ mode, the preferred button for shield tilt is MX because it allows greater movement with the shield. Shield tilt works correctly in all 4 cardinal directions, but rolls on any intercardinal input. This can be circumvented by inputting left or right first, then up or down, which will allow for the shield tilt. However, if you invert the order and input up or down first, then left or right, it will roll. It will also roll if you input both directions on the same frame. Furthermore, in my testing it appears it will roll regardless of which direction is inputted first if they are inputted too fast.
This not only makes it more difficult to shield tilt but also can cause accidental rolls when trying to wavedash out of shield. Additionally, this prevents the method of shield dropping used in Melee (Axe method) as you will roll instead of being able to shield drop. This means you have to change which shield drop method you use between games which is not ideal.