I believe we are running into this same issue running Unity Connection 11.5 - Looking at the syslogs via RTMT and wireshark captures we can see when Unity will drop a ICMP request.
Solution right now is to preform a factory reset via SSH on the phone - CLI command: reset factory
Once a factory reset has been issued the phone stops sending "ping flood" requests to syslog file - Look at Utils firewall ipv4 list to view iptable configuration for ICMP you should see log "ping flood" on X amount of burst.
We also see ICMP data using UDP traffic so phone is sending what looks like a ICMP request but contains data - I believe this RTP data since a ghost call is being made from phone to Unity server during this time.
