That is : one interface has absolutely zero "DHCP Client Options" set in its "IPv4 Configuration" in Unifi's GUI, i.e. the ISP behind this WAN interface does not need any specific DHCP option, neither v4 nor v6.
Then, dhcpv6-mod will not do anything to the WAN interface because it has no DHCPv4 option : the standard odhcp6c client will be launched as Unifi does normally, with no DHCPv6 option generated, and no configuration file used.
Then, you are OK with only one config file, because dhcpv6-mod will only apply the dhcpv6.conf config to the interface that has at least 1 DHCPv4 option set.
In that situation, for each interface, dhcpv6-mod will first look for an interface-specific configuration file precisely named dhcpv6.conf.<interface>[.<vlan>] : for example dhcpv6.conf.eth3, or dhcpv6.conf.eth3.832, depending on your VLAN configuration.
If that file exists, then its specific configuration will be applied (only) to the named interface (eth3 or eth3.832 in the examples above).
Note that this interface-specific config file can be empty (then, no V6 option generated), but at least it has to exist if you want to avoid the default configuration.
Instead of creating a completely empty configuration file, it is sometimes useful to use just the two lines below, in order to be completely standard (i.e. no options requested from DHCP server, and no particular odhcp6c command option passed), as the defaults could be undesirable :
dhcpv6_request_options= # otherwise defaults to 17,23,24
odhcp6c_options= # otherwise defaults to "-a -f -R"If there is no interface-specific configuration file, then dhcpv6-mod will take the default one, dhcpv6.conf : so you must at least create a specific configuration file for one of the interfaces. The other will be configured with the non-specific config file.