> In IP discovery GUA is prioritized over ULA. And if the GUA goes away or changes regularly it leads to connectivity issues.

Sure, but you're generally not going to get both addresses associated with a single host unless you either hard-code them or use DNS. For any non-stupid network, source address selection for reaching LAN hosts doesn't really matter. And for many-to-most SOHO networks, you're going to be running dnsmasq, which will quickly set you right after any GUA change.

And like, the thing to be noticing here is that source address selection priorities for both IPv4 and IPv6 seem to give us the same actual behavior. Consider a pair of hostnames served up by BIND 9 and some applications running on bog-standard Gentoo Linux whose '/etc/gai.conf' contains only comments and blank lines:

  # getent ahosts dumbhost.invalid
  1.1.1.1         STREAM dumbhost.invalid
  1.1.1.1         DGRAM  
  1.1.1.1         RAW    
  192.168.30.30    STREAM 
  192.168.30.30    DGRAM  
  192.168.30.30    RAW    
  # ping -n -c1 dumbhost.invalid
  PING dumbhost.invalid (192.168.30.30) 56(84) bytes of data.
  From <redacted> icmp_seq=1 Destination Host Unreachable

  --- dumbhost.invalid ping statistics ---
  1 packets transmitted, 0 received, +1 errors, 100% packet loss, time 0ms

  # curl -v dumbhost.invalid
  * Host dumbhost.invalid:80 was resolved.
  * IPv6: (none)
  * IPv4: 192.168.30.30, 1.1.1.1
  *   Trying 192.168.30.30:80...
  * connect to 192.168.30.30 port 80 from <redacted> port 47140 failed: No route to host
  *   Trying 1.1.1.1:80...
  * Established connection to dumbhost.invalid (1.1.1.1 port 80) from <redacted> port <redacted>
  <snip>
  # getent ahosts dumbhost6.invalid
  fd00::1         STREAM dumbhost6.invalid
  fd00::1         DGRAM  
  fd00::1         RAW    
  2002:db8::1     STREAM 
  2002:db8::1     DGRAM  
  2002:db8::1     RAW
  # ping -n -c1 dumbhost6.invalid
  PING dumbhost6.invalid (fd00::1) 56 data bytes
  From <redacted> icmp_seq=1 Destination unreachable: No route
  
  --- dumbhost6.invalid ping statistics ---
  1 packets transmitted, 0 received, +1 errors, 100% packet loss, time 0ms

  # curl -v dumbhost6.invalid
  *   Trying [fd00::1]:80...
  * connect to fd00::1 port 80 from <redacted> port <redacted> failed: Network is unreachable
  *   Trying [2002:db8::1]:80...
  * Host dumbhost6.invalid:80 was resolved.
  * IPv6: fd00::1, 2002:db8::1
  * IPv4: (none)
  ^C
  # ip addrlabel list | sort -hk 4
  prefix ::1/128 label 0 
  prefix ::/0 label 1 
  prefix 2002::/16 label 2 
  prefix ::/96 label 3 
  prefix ::ffff:0.0.0.0/96 label 4 
  prefix fc00::/7 label 5 
  prefix 2001::/32 label 6 
  prefix 2001:10::/28 label 7 
  prefix fec0::/10 label 11 
  prefix 3ffe::/16 label 12

Notice how actual application behavior doesn't match what folks in this thread are saying that source address selection dictates. Android behaves the same way, and -weirdly- my OpenWRT hosts have the exact opposite behavior and prefer using global addresses to ULA or 1918 ones, regardless of IP protocol version.

Anyway. If he put both IPv4 GUAs and 1918 addresses in DNS or his '/etc/hosts', he'd be having the same problem he's complaining about here... depending on -I guess- the OS he's using and how his resolver treats the ordering of entires in '/etc/hosts'.

This isn't an IPv6 problem, this is a "Things break for a little while when I use addresses to communicate with a host that can change out from under me instead of addresses that I control and those addresses happen to change." problem.

> ULA work well in combination with DNS though.

Yep! Most everything works great with DNS. Remembering IP addresses is for the birds.