I glued the two resistors previously mentioned to a piece of tufnol and milled them down with a carbide cutter.
Pic is here
The slightly damaged one "felt" harder to mill. Unfortunately I can't now tell you whether it was the low-Z one or the hi-Z one
But the texture looks identical, and there is no obvious internal damage in either.
So how could they be so very different in their behaviour? Could of course be the composition, or it could be the hi-Z one had microscopic cracks in it.
You can see the thin nonconductive coating on the outside, which explains why one cannot get a resistance measurement along the resistor.
Tomorrow I am going to pull out the top 6 plugs which have been in 2008-present and measure them at both low volts and with the insulation tester. With 6 years between the two purchases, the result will be interesting...