broken multicast in 3.12.6 (and probably 3.12.5)

Hi, list!

I've updated my virtual network (in Xen) to 3.12.6 few days ago and start 
encountering strange behaviour: random machins lose access via IPv6 ULA 
addresses to random others, until the moment, while losed machine itself 
pinged machine that search for it. IPv4 and link-local addresses worked okay.

I've downgraded all of them to 3.12.5 and all going to work properly until 
tonight: two machines mutually losed each other (even mutual ping doesn't 
help) until I rebooted both of them.

So,
1) I guess it can be some bug in 3.12 or hardened patches for it
2) Doesn't enybody else experienced same behaviour?

-- 
Best regsrds,
mva