I had a partial case head separation using a 1.8oz buffer

[Fess]

When the case mouth contacts the start of the freebore, there is no space for the neck to expand and release the bullet.

One can see much more, even double, the pressure increase we get with no jump when the mouth is pinned.

I wonder if the Grendel is unusually sensitive to this issue. WAY back when the issue of Quickload predicting overly high pressure for the Grendel was first raised, a plausible explanation was that on this cartridge, the base of the bullet is so close to the primer that the ignition of the primer started the bullet moving. This resulted in a larger effective case capacity before much of the powder had started to burn. If the bullet is pinched in place by the neck hitting the beginning of the freebore, the pressure increase would be even greater (combination of smaller case capacity and higher bullet clamping pressure).