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

[JASmith]

And another post directly addressing Lapua brass and how it articulates with the chamber under initial peak pressure, as well as during extraction:

Note that Bill A. said that the correct cone geometry will allow pressures in excess of 68 ksi. This happens to be in the range of published yield points for cartridges brass in tension.

Pressures higher than the yield point causes brass to flow. Significantly higher and the flow is fast enough that a hole opens in the unsupported part of the brass. That allows a jet of hot gas to fly through the action. This is not good for the action and, occasionally, the shooter.

Brass moving at lower pressures, e. g. a few thousand psi, particularly when the case obdurates the chamber, is almost entirely elastic. The elastic behavior lets the case spring back to allow extraction. There is, of course some plastic movement because the necessary clearances allow that much movement., which is why we need to size cases.

That tiny bit of plasticity also lets the case shoulder to move forward resulting in stretching. That is one of the reasons we pay attention to headspace. Avoiding bumping the shoulder unless needed to allow bolt closure increases the number of firings before the case separates at the base.

(Note that premature bolt opening n ARs gives similar symptoms but with a single shot.)

Also, aggressively squeezing the brass neck and shoulder region work hardens the brass. This results in longitudinal splits through the neck and, sometimes, the shoulder.