On iOS, using the code in the UDP socketbuilders causes the sessions tool to not join with iOS devices. Giving the error EADDRNOTAVAIL. If the user falls back to the other code function that uses the interface within the address structure, the multicast command works properly.
This is very likely due to either the interface not being copied over from the group address if it has an interface specified and the interface address does not or due to a structure conversion error. Either one would cause this to fail.
It's very likely that the cause of this is the interface address, as by default in the socket builder, it looks like the interface is set to 0, which you can't do on iOS, as it is an explicit error (differentiating behaviors).
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-77999 in the post.