While trying to debug a Samba problem I wanted to grab core-dumps of running winbindd processes, I then noticed that (as root) giving these commands:
# kill -STOP 49049
# gcore -c /var/cores/winbindd.49049 49049
causes FreeBSD 11.2-RELEASE-p5 to reboot immediately. Tried it on two different systems (one old HP D380 G5 and a new Dell PowerEdge R730xd).
I think the "-s" flag to gcore also causes this effect.
I could not reproduce this on 11.2-STABLE amd64 at r340464 (Nov 15). I did:
$ sleep 9999 &
$ jobs -l
 + 698 Running sleep 9999
$ kill -STOP 698
 + Suspended (signal) sleep 9999
$ gcore -c sleep.core 698
Peter, can you try that, using "sleep" specifically? If that still reboots your system, this is probably fixed in 11-STABLE.
By the way, did you get a vmcore dump in /var/crash?
This was fixed in stable/11 by r340290.