I have created a listed called "test" and saved this as my default (via settings menu -> set as default lister).
In this "test" lister, I have remove some columns (Type and Attribute) and added Ext. I then sorted the listing via Ext.
I closed DOpus (right click on icon in task bar and select Exit) and then ran the program again. Lister that opens up is not displaying the same layout as "test".
Changes that you make won't be automatically saved unless you have the Update Default Lister automatically when closing a Lister option turned in in Preferences / Listers / Options. So either turn that on, or after making changes manually update the default Lister using the Set as Default Lister menu item.
You are not saving your "test" layout as the default lister. You are saving the current settings as the default lister.
Although those settings happen to be loaded from the "test" layout you are still just saving the settings into the default lister. There is never any recorded connection between "test" and the default lister.
If you then change "test" it won't change the default lister. The default lister will still use the settings which were saved into it.
It's like loading a text file and then saving it as a new name, then modifying the original text file. The copy won't be changed.
To change the default lister you have to re-save it, either explicitly via the Set As Default menu item or implicitly via the Update Default Lister automatically when closing a Lister option in Preferences.
If you want to use the "test" layout instead of the default lister then you can set Opus to open that instead via the Launching Opus part of Preferences. The default lister doesn't have to be used for anything so if it's getting in the way you can tell Opus to use something else.