Spilt/Merge function in Program window

Why can I use the Spilt/Merge function in recent Premiere Pro projects with the Program monitor, but I can’t with older projects and have to use the Source monitor? I’ve read in multiple places that the text is dynamically generated from timeline edits and the split/merge tools will be unavailable. But this doesn’t explain why this isn’t the case with my most recent projects that have timeline edits.