I hate to repeat myself, but isn't it the case of an imperfect screen/preview rendering? Meaning that if you export the image, at the same stage where you can clearly see noise/posterization, as a tiff file and open it in PS, the artifacts would not be there?
The artefacts I am discussing are clearly still there once a 16-bit TIFF has been exported to PS. I zero the colour noise slider from its default of 25 before I start work on a file, because I want to carefully monitor any problems of this sort as I go along. I do my sharpening and noise reduction as the very last step in the process, before I export a TIFF or make a print. In B/W we tend to make pretty severe adjustments and manipulation of the image, probably more than one would in colour, at least to the luminance values. As in the wet darkroom, the "negative" is just the starting point. And all of these adjustments can lead to unexpected image degredation and unwanted aretefacts, but often only in a relatively small part of the picture. Because these artefacts can only be seen at 100% in LR (as with sharpening and NR), with big files like my 39MP ones, it is very difficult to keep track of what is going on, because you can only see a tiny part of the image at any one time at 100%.
Admittedly, with my 3FR files you won't see a problem in a 10x8 print. But if you print a big one you will see them alright. And our aim should always be to produce the best possible quality, surely?