Source URL after editing document metadata doesn't redirect, tries to open document instead

I have a document library that I have created a few custom forms for. In these custom forms I have set the Source to a URL of which I would like the user to return to after they save their changes. Instead what is happening is that after the user makes the save, they are prompted to save the file locally. If you cancel this the page doesn't do anything and it acts as if it doesn't save (it does save, but the user doesn't know).

Why isn't Source working with this? What do I have to include in order for it to not try and save and open the document?

This doesn't occur on the default EditForm.aspx, only custom edit forms.