Did you get this resolved?

I've got a similar issue observed with 902/903s. After much testing it appears to only happen if I've got IGMP snooping disabled.

DrayTek have not fixed this issue and have shown no indication that they ever will. I'm assuming that it is present across most, if not all, of their devices that support VLANs.

This same problem exists on their latest kit, too.

I have a 1062C that is leaking v6 RA packets across VLANs. There's an open support case but now I'm being told it's fixed when it isn't. Very poor on Draytek's part.