I have such options:

[Option("fm",  Separator=',', Default = new[] { FMode.S })]  
public IEnumerable<FMode> Mode { get; set; }
	 
[Option('q')]
public bool q { get;set; }
	 
[Value(0)]
public IList<string> Files { get; set; }

If I run my application with --fm D,C a.txt arguments, it says that option 'fm' is defined with a bad format. If I put any option between 'fm' and the value list, for example, --fm D,C -q a.txt, it is parsed successfully. You can check it in a fiddle: https://dotnetfiddle.net/yt5L9K