Sorry, this is actually by design. The settings in VisualGDB Project Properties are shared between all CMake targets, while the command-line arguments normally make sense only for a specific targets.
Hence VisualGDB would normally expect you to specify the arguments via the VS Project Properties for each target (i.e. item with the console icon in the Solution Explorer) separately and will automatically substitute them to the $(SelectedCMakeTargetArgs) variable based on the currently selected startup target. This eliminates the need to change the arguments when debugging another target in a multi-target project.