Wild Guess with No Evidence: All of those manifolds come from the same foundry.
The only aftermarket iron exhaust manifolds I've bought have been Dorman-branded. Every one has been a victim of crappy casting/machining techniques. Every one has been Chinese. I'm told by my favorite automotive machinist that the Dorman "Vietnamese" manifolds are
even worse than the Chinese-sourced units.
The Dorman Trailblazer Six-Popper manifolds are all warped when they come out of the box. Theory is that they're machined when still-warm from casting, then the manifold "settles down" as it cools, and warps from residual stresses. I've never had one crack like the Genuine GM manifolds are infamous for, but they've leaked after a couple of years. The gasket surface is rough--high-speed machining--they look like they were machined by kicking them across a concrete floor by ten-year-olds until they were "flat".
Here's a photo of Dorman "Corvette big-block" replacements, complete with gigantic iron booger on driver's side manifold, hanging out into the exhaust flow:
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This is a manifold runner from the passenger's side manifold, 674-505.
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Note that the gasket surfaces have been re-machined by my guy. They were unacceptably rough out-of-the-box. When buying any Dorman manifold--and by extension all the other aftermarket iron manifolds--you'll need to check the casting for defects, and the gasket surfaces for flatness and smoothness, and don't be surprised if they have to be touched-up with a die grinder and rotary file, then planed before you can install 'em.
Humans have been casting iron for more than a thousand years. You'd think they could take some pride in their work instead of churning out junk.