Does Android ignore DNS servers with IPv6 ULA addresses?

[TheDiveO]

(This is my first post in this group, so please bear with me if this is the wrong group to post my question; I would be grateful to be given pointers where to post my question then. Thank you!)

I try to deploy Android devices in IPv6-only networks. One example is an aging Google Nexus 10 tablet with Android 5.1.x, another is a Samsung Galaxy Tab S running Android 6.0.x. The IPv6 networks are larger island networks without any Internet connectivity. Some of these islands use GUA prefixes -- for instance, 2001:db8:: ;) --, others use ULAs -- from the fd00::/8 block. The DNS servers use IPv6 addresses within these GUA or ULA blocks. IPv6 routers then advertise these DNS server addresses through RAs with RDNSS option (RFC5006).

The Android 5.1-based devices work without problems: I can see how they SLAAC their addresses, regardless of ULA or GUA ranges. They can access DNS perfectly, using the auto-configured ULA or GUA addresses.

But various Android 6.0-based devices fail to work correctly in these IPv6 networks. They cannot resolve any DNS names when the DNS servers have ULA addresses. These Android 6.0 devices correctly autoconfigure their IPv6 addresses. Using a diagnosis app, I see these devices also learning the DNS server ULA address, but this server address disappears a few seconds later. In contrast, the Android 5.1 devices keep DNS server ULA addresses.

Is this a bug? Or is it a feature, maybe some dirty hack to handle some broken networks? Is there some issue report on Android 6 not accepting ULA addresses for DNS servers?

Best regards,
TheDiveO