This is happen to certain area of UniFi user, when TM side think you have IPv6 connection dropped, they dropped your IPv6 allocation.
2 possible solution on your side
1. Just reboot and hope it works, for like few days before IPv6 down again.
2. Just reboot until you allocated different range of IP address, like 175.x.x.x to 180.x.x.x, and got a working IPv6.
Until the whole network for your area is down, then your IPv6 will fixed magically.
I guess will be
1. A reboot for TM device fix it, should be the device running too many years, can be 1 year? too long I can't remember la.
2. TM take the network down to fix the configuration issues on IPv6
Yes, lot of times, whole network down, then up again, found out IPv6 stable again.

Before that guessing it is Modem or Router broken because fine for 1 year, then happen like another year, but then TM network down, and up after few hours, fine, no more issues on IPv6 side.
i dont have auto option on get ipv6 address section. which is i should choose? slaac?