This might be fixed in 4.3.16, though I can't tell for sure without seeing the original packet capture. WiFi routers try to send multicast ethernet frames as unicast if possible and these packets were not delivered to the guest correctly. #10019 has some more details on this. In particular ARP requests from the WiFi router are affected by this problem - though the router eventually gives up and send ARP as real broadcast and gets back the reply restoring connectivity until the next expiration of ARP cache entry for the guest.
