IGMP Snooping should not affect other L2 multicast traffic - this traffic should be flooded with the VLAN. If this is not happening, I would consider this a product bug. You said the SW release you are running was 12.4 (not possible, no such code for EX switches) and 12.3. If 12.3, which R release? I'd suggest you just load 12.3R12-S6 (look under Junos SR releases), and see what happens. I am not aware of anyone else reporting similar operation with 12.3R12[-S6] SW on EX2200/EX3300/EX4200.
If issue remains, a TAC case is probably going to be needed to make any further progress. I would sort of assume you already have a TAC case open?