Odd spark plug behavior

Topeka, Kansas, USA
1998 Chevy Tahoe
Just had my plugs changed, 1998 Chevrolet Tahoe, 5.7L V8. Am really curious what the wise ones among us might have to say. Most of the removed look like this (all of them AC-Delco 19256067 41-993, reportedly Chevy default recommend iridium for this vehicle):


But one of them, is the below, the electrode looks like it is retracted into the ceramic collar. It's not worn down, it's as if it were shoved in or sucked in somehow.


Anyway, I'd be most thrilled with any thoughts. I've heard one can tell a lot about an engine by way of its used plugs. Also, in case of curiosity, I had the above replaced with NGK Iridium IX, TR55IX 7164, they are advertised as being especially fouling-resistant which is a good idea for me and my projects.

Can’t say I’ve ever heard of a plug retracting the electrode I’d suspect maybe a bit of pre ignition and it’s just plum warn out was it gapped slightly different from the rest when fitted. If you’ve fitted the same plugs again I’d consider checking halfway between service intervals / replacement and see if you can spot and difference on the same cylinder but other than that not a scooby I’m afraid
It is altogether strange. I'll see if I can get a hirez super close up using my scanner soon (without scratching the platen hopefully). It really does look retracted / compacted (?!) and not worn. No strange marks anywhere. Very odd.

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