As reported here: https://lists.berlios.de/mailman/listinfo/openimscore-users

And Franz reasoning:
The reason for the rejection of CANCEL by the P-CSCF is the missing Route
header field (Route: ) in the CANCEL
request. Without this Route header field the P-CSCF tries to process the
request as an originating request.

The logic for the CANCEL processing in case of canceling a pending request
because of forking in the S-CSCF is missing in pcscf.cfg. I will try to
think about an adaptation of pcscf.cfg for this case. Maybe Dragos has
another proposal. But I think that's the reason.