I use the older 28 and 35 Nikon PC lenses and have noticed that they are hard to focus when shifted because the center of the focus screen starts to black out.

So, maybe it has something to do with a slightly lower light level, or, the angle of the light when it is striking the focus sensors?

I would think that longer lenses such as your 85 would have less of this issue. If the focus assist works, use it.