I'm using internet from 2 ISPs, both of them offers PPPOE and Dual stack for IPV6. One of them have a problem with IPV6 configuration from their NOC, I tried to explain but got refused by them because I'm just a normal user. Recently, a company implement ipv6 for their network and got a problem like me, so they request this ISP to investigate. Finally some missing config filled out and I got IPV6 like from nowhere. So I got 2 IPV6 prefix, and want to make a LB but figured out it is impossible for mikrotik right now. This is just a story behind what I'm trying to do.

Anyway, I just have 2 IPV6 to use at the same time for couple months. And my router (hap ac) was up like 20-30 days before it got rebooted by watchdog. I've never figured out what kind of watchdog got me there. So I didn't get much chance to monitor ipv6 from both ISP. But recently, everytime I reboot my router, one of two ipv6 address that I added get deleted. It happens everytime the router reboot. Is this a bug or some features? I tried frimware v6.45.9 - 6.47.

You can print your configuration first.
/ipv6 export

I have just figured something out. If I leave the address look like ::/64 for both, one of them will disappear, but then I add address to one of them like ::192.168.1.1/64 then it will be fine.