Can Android be used in IPv6-only networks? What level of IPv4 faking is necessary?
So after some more testing, using an especially made Brouter using a Raspberry Pi Zero W, I now know from my tests that Android 5.x, 6.0/6.1, and 7.0 in fact do support IPv6-only networks. However, the caveat is that as soon as Android neither gets an IPv4 address nor an IPv6 global address, the network manager will declare this network unusable and will disconnect from it.

The reason why I was experiencing such disconnects was because I was connecting my Android devices to a test network, where I shut down the link-local router from time to time for fixing, and only after some delay restarted it. And during these spans without IPv6 RAs advertising global prefixes and without IPv4, Android declared my network dead and disconnected.

Some more background information gained over time:


Some Android versions are allergic to ULA DNS server addresses, and only deem GUAs proper. However, the same Android versions happily accept ULA prefixes. The symptom then is that these Android devices keep connected to an IPv6 network using ULAs, but cannot resolve any DNS names after some seconds. Using one of the better diagnosis apps I notice that these Android versions first learn the DNS server address, then remove it after around 15-30 seconds. It seems as if newer Android versions 7.x are less rigid in their expectations at the moment.
Some Android versions react allergic to documentary addresses, that is, the 2001:db8::/32 address range. They are ignoring these addresses, especially for DNS servers. More recent Android versions 7.x seem to be tolerant of using the documentary address block in networks.


In the end, the safe side is as follows, unless you can avoid especially Android 5.x, but also some 6.x releases for sure:


ULAs fc00::/7 are evil ;) Please note that this includes the fd00::/8 often taught as "best practice" in IPv6 courses, so be careful.
documentary IPv6 addresses 2001:db8::/32 are evil too.
stick to real global addresses, even if they're faked.