If igmp 3 is a requirement then enable IGMP 3 on the 8900 ... I dont believe it was fully supported on the older code rev you were running and now that it is available on 15.3 you will see those messages because the switch can support it...
Trick will what affect will the 8900 have in learning and snooping the IGMP v3 packets? Where is the packet being sourced from? STB, Router, IPV6 announcement, server or pc? Odds are it was always there and because your core switches did not support V3 then all they did was ignore and pass it through.... You really need to know where that query is coming from to unravel this puzzle I would guess... good hunting