It's due to the Nylock patch. Every installation degrades it since it just causes a friction fit. YHM also sells the same screw without the Nylock patch and you can just use your own threadlocker. > https://yhm.net/buttcap-screw-without-nylock.html
A follow-up question:
Do milspec/issue rifles use the nylon spacer, now? For some reason I thought they used aluminum...
The buttcap spacer has been plastic since 1988.
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