This might be fixed in 4.3.16.
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.
