This is a regression tested in //UE4/Release-4.27 CL18319896
Screen Percentage does not behave correctly with SceneDepth Post Processing.
The Screen Percentage is set to 50, and fills the viewport
The Screen Percentage is set to 50, but the viewport displays both 100 and 50
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-163779 in the post.