2002:db8::1

Ah, so this is in the range assigned to 6to4 (2002::/16). Both Windows 10 and Android seem to treat it specially, [0] but Linux does not. So, I changed the AAAA record to 2400:db8::1 and re-ran my tests. I found out:

The 'getent ahosts' output remains unchanged.

When dealing with IPv4, all Linuxes, Android, and Windows 10 intermittently switch between preferring the GUA or the 1918 address. I seem to have gotten unbelievably (un?)lucky when testing v4 on Android, Gentoo Linux, and OpenWRT the first go around.

When dealing with IPv6, Android and Windows 10 always preferred the v6 GUA to the ULA. Every Linux I tried -and I tried several, each from different distros, some with and some without systemd, spread across years of releases- always preferred the ULA to it... except for OpenWRT, which always preferred the GUA. The only way I could get OpenWRT to prefer the ULA was to remove its GUA address but keep its v6 default route in place. Odd behavior. I wish I had a Steam Deck to see if "prefer the GUA" is something that appliance vendors tend to do, or if it's just a dubious OpenWRT customization.

The IPv6 behavior of all the OSs I tested is far better than the v4 behavior... predictability is always good. But it is remarkable that even many years ago, Linux was doing the sane thing for v6 address selection, while its commercial cousins and competitors -apparently- hadn't [1] bothered to think through how best to arrange things for "residential" users.

[0] ...even when no 6to4 addresses are configured on the host!

[1] ...and still haven't...