How do hydraulic flat tappet lifters fail?

No bent rods, all are perfectly straight.

****, i'm trying to remember how I even measured the preload. You'll have to forgive me, this was the only time I'd done this, and it was several months ago. I used the Comp 7702 adjustable rod, and I remember there was a video I followed that said how to do it. I remember I extended the rod until there was zero lash, and then extended it a bit more to give preload, watching the line on the rod because each revolution was a certain number of thousandths. Then, I took that measurement and it turned out to be the stock rod length (much to my surprise).

You mention lack of oil, what about decreased oil viscosity through overheating? When I primed the engine, both heads were flowing very well.