In theory, a PCP could diesel in the air space behind the projectile, before the projectile starts moving, and before that space expands. The high pressure air rushing in is going to compress the ambient air already behind the projectile, and heat that space for an instant before the projectile has had time to get moving. As soon as the projectile moves, the space behind it expands and then after a very short time, the TP (and flow path) acts as a flow restriction. Both of these factors cool the air below ambient temperature; thus cancelling any heating that occurred as the poppet opened. Now, the air rushing past the partially open poppet probably causes enough pressure drop over the valve to cool the air in the "heating period" I am suggesting might exist behind the stationary projectile that is yet to overcome its stiction in the chamber. Followed by inertia.
Has anyone had a PCP shoot faster and louder while generating a burnt oil smell (else it could just be the poppet valve sticking open occasionally) ? I have not heard of PCPs dieseling on "firing". If you could provoke a PCP to diesel on firing, it would seem to occur in the barrel, rather than in the air tank or plenum, where the air expands and thus cools on firing. The barrel is strong because it has only a small diameter hole down the middle, so barrels are typically not damaged by dieseling in springers, for example.
Yes, PCP barrel walls are often thinner, but just go and look at the pressure rating for thin walled hydraulic lines, made from annealed steel so that they can be formed by bending to fit a given application. Or look at the unsupported rim thickness of a brass .22 rimfire case. Small tubes don't need thick walls to stand high pressures.
It is the comparatively large bore of a PCP air tank that makes it so fragile, and its large internal volume that stores so much energy, that makes it pop "so easily", and make such a mess when it lets go.