savecore: operation not permitted

From: void <void_at_f-m.fm>
Date: Sat, 26 Nov 2022 18:19:19 UTC 
On 13-stable (n253148), am seeing in the console at startup

savecore[1415]: /dev/ada0s1b: Operation not permitted
Writing crash summary to /var/crash/core.txt.3.

It's enabled in rc.conf:

# sysrc savecore_enable
savecore_enable: YES

# dumpon -l
ada0s1b

What's broken?

tia,
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