Forgive the Workspaces in the images, just using this as a demo. 
All the Global variables for a Workspace, are only available within that Workspace.
Those variables are not globally accessible across all workspaces.
If the Global variable is available in the workspace where you’re accessing it using pm.globals.get('var_name') then it should be resolved correctly but if it’s not part of that Workspace, it would be undefined.