The -mode concatenate option sets a set of default values that are not set by the MontageSettings constructor. The MontageSettings constructor also sets a set of default options. I will add a constructor in the next version of Magick.NET that will allow you to specify the mode.
I think your problem will be solved if you set montage_settings.Geometry to null.
I think your problem will be solved if you set montage_settings.Geometry to null.