Yeah, I'm thinking something else is going on, as I'd be surprised that detonation did that.
You said its like the clamp load wasn't there, and I think you're on to something. I think 318WR said to verify that the dowel pins aren't holding the head off the deck. That's worth verifying.
I've seen bolt holes that weren't tapped to the bottom and the bolt wouldn't go all the way down because it was bottomed in the threads. That's something to look at as well.
The other thing I keep forgetting to ask is if the block and heads were decked or if one or both were not decked.
Something is wonky.