That is very interesting, Kor. Thanks for sharing.
It is indeed true that the reappearance of a bone is a pretty rare occurrence evolutionarily. However, there is precedence for 'dormant' genes or inactive genes to become active due to mutations or epigenic interactions. The periodic reappearance of a "soft tail" in human infants from time to time is indicative of this. It seems that the reappearance of the pisiform had enormous adaptive advantage and thus the trait was selected for evolutionarily.