Hmm it seems that when denoiser is used with render selected, or at least when it is used in viewport selection mode (didn't try other modes) with the previous rendered image still in vfb (the most useful scenario), it applies that specific denoising (noise pattern\noise normals passes - not sure how they are called) which is calculated specifically for those selected objects, to that whole previously rendered image, thus, probably, because of different noise pattern\pass count, messing that image quite considerably.
+ Something fishy happened to post RC's .exr dump\render history .exrs - I cannot open those of them which contains denoiser passes anymore - photoshop and picturenaut (which I use for quick previews of all 32 bit images) are both showing me this
http://c2n.me/3xPVHMo - which has never happened before and I use exr dumps for almost every rendering I do so I should have noticed that - it only happens to vfb dumps containing denoiser passes and there's no difference which 3ds max (2016 or 2017) was used or which scene was rendered. Tried to open those on all of my pc's with the same result.
Not sure exactly which release begun to behave in such a manner but the last .exr containing those passes and the one which can be opened is saved on 25 04 2016 (after that I was preoccupied with some archicad involving work and didn't render much until yesterday) so it seems that was some of the latest pre RC builds as I tend to install them on the day they are released.
edit: did some reinstalling going backwards from RC5 and it seems that RC2 is the last version which saved .exr dumps with denoising pass which could be opened by Ps or anything else.