I suspect that it has to do with VVT and that it is not moved to an engine brake solution. That is it is more like a diesel when coasting until you employ the Jake as far as engine braking. On the 3.6 it might be for pollution reasons or they just did not add effective engine ...
The VVT sprockets, chains & tensioners, etc can fail. Always diagnose first. There are discussions about cam swaps. I have not done one, but it seems that more part changes are called for. Proceed with Caution! Hemi Timing Chain Defect ...