mDNS uses a link-local multicast address (184.108.40.206) that cannot be sent off-link, so you cannot route mDNS packets.
4. Local Network
Control Block (224.0.0/24)
Addresses in the Local Network Control Block are used for protocol
control traffic that is not forwarded off link. Examples of this
type of use include OSPFIGP All Routers (220.127.116.11) [...
I'm adding an answer to the question as the supplier found the solution to the issue we were experiencing. I'm merely adding here his explanation, it could be useful to anyone else incurring in this event. Thank you again!
[13 April 2021 12:31]
I found the solution that you need to add the Virtual MAC address on one of the interfaces of both EDR.