I haven't checked with wireshark if this is effectively happens, but reading the code I'm understanding this:

When a media is shared and a 2nd client connects, it will allocate a new UDP socket for it and set it on the udpsink in alloc_ports_one_family(). My understanding is the source ip/port of future buffers sent to 1st client will suddenly change, no?

Also there is theoretically a race I think:
1) client1 connects and addr1 is allocated for him. An udpsrc is created. The multiudpsink now uses addr1.
2) client2 connects and addr2 is allocated for him. Another udpsrc is created. The multiudpsink now uses addr2.
3) first buffer goes out of the multiudpsink, so with the packet will have "source" addr2.
4) Since RTSP clients uses the source address from udp package to know where to reply, both clients will reply on addr2.

I've talked about this with Olivier Crête, and he thinks there is no reason to allocate new socket/udpsrc for every client. According to him, udpsrc can receive from any clients, and rtpbin is capable of filtering which buffer comes from which client. Current code puts every udpsrc into a funnel anyway.