pfSense

This occurred on a completely new install, version 2.5.0-RELEASE.

Specs:
Mainboard: ASUS ROG MAXIMUS IX FORMULA
CPU: Intel Core i7-7700k 4.20GHz
RAM: 8GB DDR4
Interfaces were the mainboard gigabit Ethernet port (used as WAN) and an intel 82571EB/82571GB network card with 2 ports (one LAN, one unused).
The system was installed via USB stick.

I also replicated the issue in a virtual machine with 2 virtual interfaces. The VMs were installed via DVD image.

I initially set up my interfaces from the shell.

One of the interfaces is configured to be WAN and receive its address via DHCP

I configured a second interface to be the LAN. I configured it to have a static IPv4 address. I configured it it to have no IPv6 address. I configured it to have the DHCPv4 server active on the interface.

I left a third interface entirely unconfigured.

I then went to the web GUI and continued configuration on there.

I clicked through the Setup wizard, adjusting some configuration like timezone but leaving the interface configuration as is. (It retained the values i already set from the command line.)

I then went into the settings of my LAN interface to rename it from "LAN" to "MGMT"

When saving that change, i received an error message that the interface configuration can not be saved. The exact message:
"The following input errors were detected: * The Router Advertisement Server is active on this interface and it can be used only with a static IPv6 configuration. Please Disable the Router Advertisement Server service on this interface first, then change the interface configuration."
Not that i had no IPv6 address at any point.

As instructed i then went to the configuration page "Services" > "DHCPv6 Server & RA". I could not turn off router Advertisement. I was met with the error message:
"The DHCPv6 Server can only be enabled on interfaces configured with a static IPv6 address. This system has none."

This is obviously some sort of dependency loop and should be an unachievable configuration state. I never configured any IPv6 address, thus RA shouldn't have been enabled.

Workaround: I resolved the issue by assigning an IPv6 address to the interface. Then opening the "DHCPv6 Server & RA" settings and disabling router advertisement. Then removing the IPv6 address from the interface again.

Nature of the bug:
Despite assigning no IPv6 at all in the CLI configuration, and only activating the DHCPv4 server, RA was activated on the interface. I suspect this is an issue caused by the CLI configuration tool.