Hi. Intel AMT Commander does not really know if serial-over-LAN connection is dropped, so, after you reboot to BIOS, try going in the terminal menu and disconnect and connect again. After that, press a few keys to see if the BIOS responds.
When the Intel AMT computer is in the OS, you will not see anything on the terminal unless you have a serial agent running. Run Intel AMT Outpost on the AMT computer and enable the serial agent. Make sure you ahve the serial-over-LAN drivers installed. You should see the Outpost management prompt on the terminal.
UTF-8 terminal emulation should never be used, I don't know anything that supports that mode.
I hope this helps, please let us know what your results are.
Ylian (Intel AMT Blog)