VMware network serial port concentrator

I configure network serial ports to three VMs for vSPC.py serial port concentrator.

For certain VM, vSPC.py works, but for certain VMs, vSPC.py does not work.

In the failure case, when I type some characters on the remote telnet window,

the VM does reply ack, but does not echo the characters back to the concentrator,

so that the telnet session looks like frozen.

From the wireshark capture, I can see "DO 1"  (ECHO) from VM is missing during the

telnet negotiation between VM and concentrator.

Question: Is this anything to do with the individual VM's UART serial port driver?