Userland is built from git hash 8f3c71c85e.
The setting 'dumpdev="AUTO"' exists in /etc/rc.conf yet after booting 'sysctl kern.shutdown.dumpdevname' is empty.
Manually running 'service dumpon start' does populate this sysctl.
After manually starting dumpon, I can force a crash to DDB, using the documented hotkey combination.
My first thought was to revert to the commit prior to "git: 77e1ccbee3ed - main - rc: implement parallel boot": "git: f3f7b0dc065c - main - lib/msun/ctrig_test: Print the mismatched values on failure"
That did not help. The next commit candidate to revert to would be svn commit r366856.
Updating /usr/src/ to git commit https://cgit.freebsd.org/src/commit/?id=763db58932874bb47fc6f9322ab81cc947f80991 will set 'kern.shutdown.dumpdevname' again.