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

[DeNinny]

Not criticizing, but is that picture super small for anyone else?

I think early unlock still. After staring at it, to me the flap occurred after blowout as that's where the initial separation started, and saw the most pressure as the seam continued radially chewing the brass with powder, hot gas, and fire.

Sorry for resolution. You can look at OP's first post of the picture and you'll see the same thing there, without arrows.

I'm sorry but the flap could easily occur from the blowout itself, when the pressure is much higher than during extraction. Because of this, I think the blowout has a higher likelihood of creating that flap instead of the extractor pulling on it.