I have confirmed that this is only an issue with the TP Link switches. I tried a new one (fresh out of the box), same issue. I had a couple of unmanaged Aruba switches (model 1430) and tried those...

@Clive_A I've actually got Wireshark files for both the Cisco and the SG (zip attached).

@Clive_A - laptop and mic (.158) are both connected to random ports on the sg108 (and even tried a different sg108 - same result). Duplicated the same setup with the Cisco switch.

I have several of these switches in a few wireless mic racks and I'm having trouble seeing them via the manufacturer's software (Shure). It appears that all the traffic that they use is IGMP...