Hmm. I haven't run into this issue before (if it ever doesn't default to vol pedal, I just stomp it to switch to it), but just some ideas of things you could do to work around it:
First, I would say to double-press Move while highlighting the vol pedal in your chain, and make sure that it is assigned to one of the expression controllers.
Then I'd check in HD Edit to see if I can assign it through there.
Next, I'd go through and check it any other pedals in my chain is assigned to the expression controller that I want to use for my vol pedal.
If that doesn't work, I would say to remove the vol pedal from your chain, then save the patch to a new location, and try adding the vol pedal on this new patch and see if that'll work.
Good luck!