We have this issue for several weeks now, but for some reason no issue was opened yet (only sub-mentions in related issues). The symptom: the GUA of a node disappears, it is purged from the 6LBR's neighbor cache (as well as the GUA of the 6LBR on the same prefix on the 6LN's side IIRC) and even a new PIO does not re-add the address. Several issues have been found that might be related (or causing) to that:

• router advertisements have global addresses as source address (ndp: router advertisement sent with global address #5005).
• address registration of GUA according to RFC 6775 is not sending out any following NS after the initial NS.