ISX exhaust valves leaking oil

[BlizzardGuy]

A customer I work with purchased a "OEM reman, Signature CM570" from a 3rd party "surplus" engine reseller. Long story short... They sent a QSX15 engine painted red, and changed a few parts so it was suitable to be mounted in a truck.
Throughout this whole process I saw nothing but red flags and recommended against it, but he insisted on using this engine.

This doubtfully has anything to do with the subject of this thread anyways --- just backstory.

So I ended up converting the rest of the incompatible parts over (ECM, harness, fuel system, tune, etc) Got it running and driving the truck just fine
Not even 200 hours runtime later, engine oil is seeping past the exhaust manifold gaskets and turbo mounting gasket
Of course the engine reseller wants nothing to do with providing any warranty, so I was tasked with investigating.
Pulled the turbo & manifold and -- as expected -- engine oil running down the exhaust valves, 4 out of 6 ports soaked
Checked the valve seals, they are the latest revision (sit underneath the valve spring) and appear in OK shape.
The owner of the unit requested to just replace the valve seals and clean the oil residue and see what happens. I did just that - reassembled and we were good for another few dozen hours.
Same issue is still present

I've seen plenty of ISX heads with this problem, but always just expected it to be caused by valve seals.
The internals of this particular engine appeared to be actually new and OEM. As skeptical as I was, I don't actually think the builder of this engine neglected to replace what was needed properly

****
Since this time, the head was removed and sent for inspection and magnaflux by a local diesel engine shop. They claimed they could not find any cracks or issues with the valves, guides.
They replaced the seals again, sent it back.
I wasn't satisfied with this result, but the owner of the truck went ahead with installing the head and crossing fingers

Guess what.
Problem still there, and in fact, is worse than ever before

Out of ideas!