The DHCPv6-server currently does not support address delegation, it only supports prefix delegation. The DHCPv6-client supports both.
Without address delegation Windows does not receive any IPv6 DNS servers.
If it's mainly DNS you're after, RouterOS DHCPv6 server can already run in stateless mode and provide DNS. Just add new server without any pool, set managed-address-configuration=no and other-configuration=yes in /ipv6 nd and it will take DNS addresses from /ip dns and send them to clients when they ask.Without address delegation Windows does not receive any IPv6 DNS servers.
I cannot get that to hand out DNS. The router just keeps discarding the request packets, because it only supports prefix delegation. Windows does not currently support DNS via NDP.If it's mainly DNS you're after, RouterOS DHCPv6 server can already run in stateless mode and provide DNS. Just add new server without any pool, set managed-address-configuration=no and other-configuration=yes in /ipv6 nd and it will take DNS addresses from /ip dns and send them to clients when they ask.
Odd.What's your RouterOS version? It was broken this way (not sure for how long) and was fixed in 6.34.x, but I did not test if the fix made it also in 6.32.x bugfix branch.
Then there's another possible problem, but behaviour is different. When DHCPv6 server looks for DNS address to use, it unconditionally prefers dynamic (added by DHCP client) over static ones. So it works when you either don't have any dynamic DNS resolvers or have both IPv4 and IPv6 (from DHCPv4 and DHCPv6 clients), but it fails when you have only dynamic IPv4 (only from DHCPv4 client), even though static IPv6 resolver exists. In that case it sends "empty" reply packet without DNS. But again, if it rejects the request, it's not this.
Edit: I just tested it and 6.32.4 works fine too. So either you have something even older or you don't have other-configuration=yes in /ipv6 nd.