Currently the AUX Switch PDL option toggles between the low and high values of the Expression Pedal assignments. I have added your suggestion to the feature requests to be considered in a future release.
A current workaround would be this:
When you save a Preset, it saves at whatever position the Expression Pedal is currently set to. So if you want your Preset to sound like the Expression Pedal is in the middle, move the Expression Pedal to the middle and save the Preset.
Now save that same Preset to the other location in the same Bank. You should have a Bank with two identical Presets.
With this setup, after you use your AUX Switch (set to PDL) to change the sound, you can load the other Preset in that Bank and it will be like resetting the PDL back to the middle.