I've installed Win Server 2019 from an empty template I had of a fresh server. Installed Domain Services, promoted to Domain Controller, created an entirely new forest but then noticed I couldnt join nor add anymore DCs to it. Looking at the DNS records I found them to be lacking to say the least:
Notice that it's missing _sites,_tcp,_udp,DomainDnsZones and ForestDNSZones.
Did you just promote a domain controller with the name of an existing domain without first joining it to the domain? That's a wrong order! You have just created a completely new AD domain with an overlapping name.
First join the server to the domain and then promote it.
This ended up being a defective image of some sort. I downloaded a new ISO for server 2019 from Microsoft, reinstalled everything and it worked right out of the box.