When the PAWC of a layer is 0.0, and XF=1.0, the crop can take up water that is above DUL. This is bacause the supply is calculated thus -
SWSupply[layer] = Math.Max(0.0, kl[layer] * KLModifier.Value * (sw_dep[layer] - ll[layer] * dlayer[layer]) * RootProportion(layer, Depth));
You can see that sw depth, minus the LL depth (and LL depth equals DUL depth), then the supply contains the water above DUL.
The consequence of this is when the SW is in the saturated zone, the supply will be too high when multiplied by KL. I understand the maximum supply should be KL* PAWC. Thus, in saturated conditions, the crop has the potential of taking up more water than KL should allow.