systemd, as we all know, can be a real pain in the ass. Configuring ipv6 interfaces is no different. The reason here is that it changes the rules from version to version on how to setup a networkd interface.
Just be sure to not set a default gateway and let the interface do that for you. It used to work, but now it doesn't. Just don't do it.
Disabling With Networkd
There are cases where it's not necessary for IPv6 to be enabled on an interface. I did recently go through a class that the instructor said that "there's never a reason to do this," but I don't see the harm. For example, I have a server where three interfaces are IPv4 only and one that is IPv6 only, so I disable IPv6 on the three that don't use it.
To disable, add to the