> My router advertises both the public prefix from my ISP and a ULA prefix for internal LAN connections
You'd think so, but you'd be wrong. If you have a GUA and a ULA, and the printer has both a GUA and a ULA, then your host is going to prefer the GUA when connecting to the printer.
So when a router withdraws a prefix, it's possible for your computer to still have a (stale) GUA while the printer only has a ULA. Whoops, and your printing is broken.
You'd think so, but you'd be wrong. If you have a GUA and a ULA, and the printer has both a GUA and a ULA, then your host is going to prefer the GUA when connecting to the printer.
So when a router withdraws a prefix, it's possible for your computer to still have a (stale) GUA while the printer only has a ULA. Whoops, and your printing is broken.
There is an RFC in review that would fix this: https://www.ietf.org/archive/id/draft-ietf-6man-rfc6724-upda... . It'll likely be approved in 2027, so you'd be able to use ULAs some time in the 2030-s once it trickles down.