Hey @Quasar,
Sorry for the late response, I didn’t get any notification about your post (looks like I’m a newbie to using this forum engine properly 
).
I will add the log rule and enable the debug messages for igmp-proxy, and will check what happens.
Meanwhile, do you have any news about SUP-98496, or do you know v7.5 contains this issue as well?
(As I saw http://forum.mikrotik.com/t/v7-7beta-testing-is-released/161807/1 the IGMP Proxy not working with v7.7beta8 
).