It is a distinct possibility, given your description, that the problem you have is not a scanner, developer, or film issue, but actually "flare". The lenses on your cameras may actually be the difference.
The optics on your Hexar may simply be affected by or imparting more stray rays of light that eventually get to the film than the Nikon lens.
The fix may be as simple as using a lens hood or your hand to protect the front element on your Hexar from seeing stray light. Worth a try