Created a fresh jail from the simple jail creation wizard and it will not start. The jail appears to start but then immediately stops with no console log output. I never had this issue prior to upgrading from 11.3-U5 to 12.0.
One key difference is that the new jail is the new default of a clone jail vs base jail.
BTW, I used the category "Plugins" for this issue as the "Jails" category seems to no longer be available.