pfSense

In /etc/inc/interfaces.inc function interface_dhcpv6_configure

  4946                  if ($wancfg['adv_dhcp6_config_file_override']) {
  4947                          // DHCP6 Config File Override
  4948                          $dhcp6cconf = DHCP6_Config_File_Override($wancfg, $wanif);
  4949                  } elseif ($wancfg['adv_dhcp6_config_advanced']) {
  4950                          // DHCP6 Config File Advanced
  4951                          $dhcp6cconf = DHCP6_Config_File_Advanced($interface, $wancfg, $wanif);

As function loops over interfaces, if one interface has either override set then the entire file will be overwritten.
I suspect "=" should be ".="

This is in pfsense+ 22.05