[MP70] Duplicate multicast packets

I have an AirLink MP70 LTE Router.

I have a setup with two laptops connected as a LAN. When sending a multicast packet, the router is duplicating the packet, so two identical packets are received by all group subscribers.

I have tried a factory reset, and updating to the latest firmware (4.9.3) but the problem persists.

I have tried the test with two other routers (Cisco, Vocality) and they both perform as expected - with only one packet received.

Is this an issue with the MP70 router, or is there a default setting I need to change to stop this behaviour?