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.

Interesting. Honestly I like having control over it, that would annoy me. I deliberately change the DUID in dhcpcd to force my public addresses to change every so often.