Could you describe the issue you are seeing in more detail?
I have a multimonitor setup. As expected, automatic display settings toggles the resolution of the monitor that will host playback.
I experienced two issues after installing MC18 (103). The first was that my secondary monitor which is either a projector or plasma did not change frame rate to match the content. I set the default frame rate to 50Hz @1920x1080 and with MC17 the frame rate will change to 23.976 for film and 29Hz for concerts etc dependant upon the content frame rate. With MC18 the frame would not move from the default rate unless I configured my system to use one monitor.
The second issue was that in spite of correctly setting the monitor configuration, MC18 would frequently open on the primary monitor, even though theater full screen mode was set for the secondary display. My primary monitor is normally switched off and only used for audio operation.
My configuration has two zones, Video (0) and Audio (1) and is totally controlled remotely using MC core commands along with the startup command line option that you recently fixed. Zones are selected during the start up process.
I have as you may know re-installed MC17 and everything is now working as before.
I hope this information will assist. Please do not hesitate to let me know if I can offer any additional information.