If you look at the thread with the comparison of the 2018 and 2019 wings, you can see how the wake from the front wing doesn't get pushed far enough out with the Red Bull style wing. Mercedes have dropped the outer edge of theirs a bit and cut out the endplate, I assume to get just enough wake outside the car to continue sealing the edge of the floor.
The wing does generate strong vortices however, that can run along the entire length of the floor, so maybe they are looking at how that is effecting the edge of the floor for sealing.
I assume Mercedes figured out how to use both the barge boards and the front wing to seal that area of the car in tandem and as such, are having more success with it.