Newer Samsung phones are DROPPING any IPv6 packet (not just RAs) as soon as the screen is off.
(this is in the WiFi firmware so even 3rd party roms like CyanogenMod are affected).
So anything that is connected via IPv6 will be disconnected.
See this thread (the title is about ICMPv6 but later it clarifies that newer phones drop ALL IPv6 traffic): http://developer.samsung.com/forum/board/thread/view.do?boardName=General&messageId=239890
Even if they do that to save battery, there are better ways to do this and it should be configurable.
Please help and raise a stink with Samsung over this!