If two hosts have both mutually accessible ULA and GUA addresses, they will prefer _GUAs_ to talk to each other. So the connection will be susceptible to the prefix withdrawal if the upstream goes down (BTW, IPv6 did nothing sane for multihoming either).

> Are you taking about mDNS still referencing the withdrawn prefix?

That too.

Oh, interesting. My machines have not had good mDNS support, so I have been hardcoding ULAs for LAN-only traffic.