Ok now this one is surely a bug. Say you have a detailed model with SubD, it has 1 million polygons

If you link the collection of that model into a new file, in that file if you enable Simplify, Viewport Max SubD to 0, you’ll see that the linked model will not get un-subdivided in the viewport, and yet any other model local to that file will be un-subdivided. You have to save the file, close it and then open it and then Simplify will take effect and the linked model will be un-subdivided.
If you uncheck Simplify, to get all your smoothness back, the linked model will stay un-subdivided and jagged, you have to save, close it and reopen so that the unchecked Simplify take effect.