3013087 - Firewall log a connection initiated back from remote ASE host listening port to the unprivileged port on local ASE host - SAP ASE

Two ASE instances: A and B
Instance A is running on port 4900, instance B be running on port 4100
There is a firewall in between the instances.

There is a remote server connection on instance A to instance B
Connections are initiated from A from an unprivileged port to instance B on port 4100
e.g. connection from host A port 43534 -> to host B port 4100
This is expected, but sometimes the firewall log shows a connection initiated back from host B port 4100 to the unprivileged port on host A

Why is this connection from instance B to instance A happening?

SAP Adaptive Server Enterprise (ASE) - all versions

enable xact coordination, ASTC, DTM , KBA , BC-SYB-ASE , Sybase ASE Database Platform (non Business Suite) , Problem