I believe we saw something similar when we installed our first MS250 at our HQ, hanging off a stack of Catalyst 3650's. Our firewall suddenly started seeing a bunch of IPv6 router solicitation requests and that set off some alarms (we shouldn't see any IPv6 traffic).
In the end, we ended up disabling IGMP snooping and Flood unknown multicast traffic under Switch -> Switch Settings -> Multicast Settings. These were enabled by default, with no way to adjust separate settings between IPv4 and IPv6.