IPv6 lost connection for a while periodically, with Apple TV in the LAN

Hi, I am using OpenWrt as my home router, with 2 routers act as wireless AP. Openwrt utilize PPPOE to get Internet access. My ISP got IPv6 PD support and I actually need it. I set up the IPv6 on OP but occasionally I found out IPv6 connection problems, client in the LAN cannot ping IPv6 hosts but my OP always can. On my NAS, I use Home Assistant's Ping intergration to notify me if the IPv6 connection breaks. When outrage happens, ip addr on NAS shows a fdxx address as a prefered address. I re...