Depending on your network configuration, requests may not be always answered by the host the request was sent to. For example, a firewall, router or switch may be configured to forward any traffic to TCP port 5357 to
As a result, when conducting a port scan for any host in this network, TCP port 5357 appears to be "open" for every host, but in reality, every request is answered just by one host.
In your scenario, it might very well be that port 5357 on one specific host (e.g.
203.0.113.29) is closed. nmap would still consider it as "open", because a request for
203.0.113.29:5357 would receive a response.
nmap is simply not able to tell whether or not the response came from the host or not.
How can I make sure this is what is happening?
You can use Wireshark or similar tools on
203.0.113.29 and see if packets sent to this host on this port actually arrive there. If they don't, but nmap still considers the port as open, then you are not communicating with this host.