Install puppetserver6 using pkg on a clean 12.0-RC3 system, enable the puppetserver service and start it. Monitor /var/log/puppetserver/puppetserver.log. You'll see a stream of errors relating to Puppet's inability to modify certain files and/or directories on the system and the service won't start up.
Perform the exact same actions on 11.2-RELEASE and the Puppet server starts without any issues.
I'll spend some time in the coming weeks getting a 12 box up. The motherboard for my build box is currently on my dining table, so some things need addressing first, but I'll try to reproduce this here shortly.