I have a separate condenser assembly for 35mm on my Omega II. It really only inceases the light intensity and coverage - slightly. I was wondering if the Nikon lens could be producing some internal reflections which might be producing your halo? How far out side the image area is this halo? If it is far enough, why worry about it?

Truly, dr bob.

P.S. I also have a Nikon 50mm lens from another enlarger which I can try to see if it reproduces your observations if you wish - I'll have to spend a small effort to locate and mount it, but will be glad to try....