Typically it's similar to ipv4, they try to assign the same address/prefix for the same MAC/DUID. The most common reason to lose your addresses is replacing your router. Hopefully new routers allow you to set the dhcpv6 DUID somehow...

I haven't experienced this. For me it's statically assigned but my guess is that the PON serial and/or MAC is being used or the customer ID. I think the ISPs have gotten very automated these days and everything seems to be some sort of SDN. It saves lot of labour hours in troubleshooting like customer forgetting their wifi passwords to their routers.