New here... late to the party on this, however if you see this, OP, did you have any different results this year with the same loading, did you change your loading?
I've had g success with the 130 TTSX in a 30-06. Antelope and elk have been taken with that bullet. Wouldn't hesitate to use the 150 TTSX, however experience tells me the 130TTSX would get the job done with an even flatter trajectory and more impact velocity, and it has.
With that said, your 308 Win launching the 150 TTSX at 2825 FPS had the needed velocity (as you said, perfect mushroom) with a decently placed shot on your elk, not so much on your fathers elk.
Chalk it up to the first shot not being where your father wanted and having to finish an animal hanging on to life and energized.
My deer/elk rifle is a 35 Whelen AI, shooting the 200 gr TTSX at a chrongraphed 2925 FPS. A dozen elk and a few mule deer in there for good measure with Barnes bullets, the majority being the 200 TTSX in recent years.