I suggest searching the mailing lists... It looks like that sysctl may have been removed and most searches seem to give results relevant to FreeBSD 8... so far as I can tell it was removed because it's now the defaut anyway since FreeBSD 10. So messing with that might be futile.
However there was a more recent and more relevant bug report a few years ago and it was fixed, so perhaps a regression?
https://bugs.freebsd.org/bugzilla/sh....cgi?id=188278
Perhaps file a bug report and reference that bug?