[smoothy8500]

I worked up some other test loads and was still flattening primers down to 38gr in half grain increments.

"Flattened" primers sometimes occur due to oversized/overworked brass. The shoulder possibly was bumped too far, causing a "gap" between the boltface and primer. The primer initially backs out upon firing then gets flattened as the case expands.

 

[smoothy8500]

Nosler book shows 43gr as the max w/2.800" OACL

While I agree the book lists this as "MAX", you need to see what type of rifle they used for the test. Looking at Nosler's website, they show a Lilja 24" bbl achieving 2708 fps.
https://load-data.nosler.com/load-data/308-winchester/

Using a bolt action rifle gives you the ability to go a little higher pressures than what is considered the norm in a gas operated rifle such as the M1A/M14.

Regardless, I believe the "primer flattening" you see at 38gr is due to oversizing/overworking the brass and affecting the cartridge headspace.