This is a little complicated, but let me try to explain this as best as I can...

Currently, whenever you add effects to an object or a clip, the effects are - by default - applied to just that INSTANCE of the object, rather than to the master (which would affect every instance). And this is exactly what you want for video clips and other objects, because in most cases you'll want to apply different effects to each instance of a clip.

However, when it comes to Adjustment Layers, it's a whole different story. An Adjustment Layer is BUILT from effects. And Adjustment Layer is DEFINED by the effects it contains. Therefore it makes perfect, logical sense that when you add effects to an Adjustment Layer, the effects should be added to the MASTER item, because without those effects, the Adjustment Layer serves no purpose, and has no reason to exist in the Project panel.

Or let me try to put it another way... Let's suppose you have a project where you want to have several adjustment layers each containing DIFFERENT effects. What would you use? Logically, you would use several DIFFERENT adjustment layers, not the same one. The only time you would use multiple INSTANCES of the same adjustment layer would be if you wanted the SAME effects to appear consistently at different points within the movie.

So it makes sense that effects added to Adjustment Layers should default to the MASTER, instead of to just the INSTANCE.

This would mean that Adjustment Layers in your Project panel aren't just pointless "empty shells" containing no effects - every time you start adding effects to Adjustment layers, you are DEFINING that Adjustment Layer, giving it an identity and a purpose.

To me, would all make better sense, and make the application far less confusing and easier to learn. But, admittedly, I am new to Premiere Pro, so I shall humbly defer to the wisdom of long-time users on this matter, and see if anyone agrees with me?