I like Phil's answer a lot!
Very plausible explanation.
However, as suggested in the other forum, get a borescope and take a very careful look down that spark plug hole.
You are doing this blindfolded.
The scope will quickly allow you to determine if someone had attempted to install the wrong plug with a washer, or even two, and they remain down there and interfering.
The scope will also let you observe if there are deposits lodged in the threads as Phil has experienced.
Once you know the certain cause, solutions will narrow down to the correct set.
Those are expensive heads to be evaluating by speculation as opposed to easily obtained empirical evidence.
Good luck, keep us posted, and get a set of new plugs.
I am confident that you will have this corrected quickly once the true cause is known.
Marty