First layer inconsistent across build plate  

First layer inconsistent across build plate

I've been printing on my MK3S+ with the MMU2s for a while now and am mostly quite satisfied with its performance.  However, as you can see from the attached photo, the first layer is rather inconsistent across the build plate.  This is a photo of a print near the end of the first layer.  If you look carefully, you'll notice the first layer is near perfect (in my opinion) in the lower right and upper left corners, but degrades steadly toward a band through the center of the build plate.

I placed a 12" combination square on edge on a glass work surface to verify its straightness and then placed it front to back and side to side on the build plate.  I found no issue side to side, but I did find the plate seems to fall away from the square by a few thousandths of an inch at the front of the plate.  However, that doesn't seem like it would explain the behavior I'm seeing.  Also, FWIW, this behavior didn't show up until after I installed the "+" upgrade to the MK3S and the MMU2S.  Is it possible the new bearing clamps are causing some deformation of the build plate?

Any suggestions for how to correct this issue would be greatly appreciated.

Thanks

Bill Lugg