Personally, I remember the forensic anthropologist's expert testimony at the trial that the duct tape was present on the face during decomposition. The duct tape must have been attached to some part of the decay for some time as the mandible was still in position.
Experts also testified that not many craniums at this stage of decomp are found with mandible at all, much less a whole skull with mandible in place.
Considering other bones were scattered by animals, that animals chewed on the bones, it is odd that the mandible was there with the cranium as if it were somehow attached, even though some of the skull's teeth were not.
Considering the speculation that Kronk stuck his meter reader tool in the eye socket and lifted the skull, or that he moved the skull at all, I would not expect that mandible to be in position. I have constantly wondered how the jurors got around this notion, since there was not a clear counterargument in my mind...what else would keep the mandible in place but the duct tape?