They could be walking either with their arms held off the ground or with their hands being placed on the ground (walking on all fours).
Would the footprint of the feet be the same in both cases?
Let's look at this diagram:
It would seem possible that the plantigrade footprint is an artifact of all-fours walking.
It could well have been digitigrade when the arms were held off the ground.
This would explain the plantigrade footprints of a pterosaur found (based on an instance of all-fours walking). And explain the digitigrade footprints of birds which never walk on all fours.