If:

No known debris is between washer and head or between plug and washer...

There is known "good" washer...

Knowing the plug is well and good tight enough...

And there is still leakage, knowing that it's a brand new engine, it's very possible that the head or head button was not machined correctly or the plug itself is not.

Try different plug, if problem persists, I’m going with my theory of machining problem. It’s rare but not unheard of.