Ive had this problem for a while now that IPV6 addresses don't show up in the neighbors tab when starting up winbox. They do show up when rebooting from upgrades then go away when everthing is up and running again. Most recently I had to reboot my ISP's modem. While that modem was down, the IPV6 addresses of my devices showed up in Winbox. Using Winbox 3.18 and always fully upgrade to the most recent stable versions.