JPD
Retina and Rollei user
The edges can be softer than the centre wide open, but I disagree with the assertion the Xenar is an unsharp lens per se. It's probably more likely the issue is with focus adjustment of the camera, or a need to practice using it.
The Rolleicord III and IV I had were never capable of bitingly sharpness even stopped down though they were clearly in focus. And I had plenty of experience using Rolleis. The lenses were clean, and the front and rear elements screwed in place like they should be. Other Xenars, like one on a V and Rolleiflex Automat (the first post-war model) were excellent.
The only explanation I could think of was sample variation. Since it were in focus, the rear lens group must have projected the image correctly on the film, so maybe the problem was with the first or second elements, not being ground to the exact specifications. The serial number on the front element rear element was the same. Schneider used to stamp it on the mount of the rear lens group as well. So the front and rear groups were matched.