I recently tested connecting to Debian Stretch from Debian Jessie via XDMCP and had no problems. That and the passage of time led me to look to see if a newer Xorg server was available from Cygwin and there was. I just downloaded the latest Cygwin Xorg server, version 1.20, and it connects to Debian Stretch via XDMCP just fine. Therefore, I consider the problem to now be resolved.