Why does LR4 show brightness changes after external processing in CS4?

Question

I am an experienced user of Lightroom and get excellent results. But an anomaly is puzzling me. I capture images in DNG format and download to a PC Windows 7 using a calibrated Eizo graphics monitor. Before exporting and delivering a processed image I ensure that there is no unacceptable clipping showing on the histogram. I export 16 bit files which are only converted to 8 bits immediately prior to delivery. Recently I have adopted the procedure of importing a copy of the final delivery image back into LR4.3 for better library management. (There are often keyword changes or other refinements in the delivered image) Occasionally I notice that some pictures are noticeably brighter than the existing LR processed masters which formed the basis of the above operation. So I did some tests.
I selected an image with wide tonal range, adjusted it in Develop module to meet my 'no unacceptable clipping' rule and exported it as described above into CS4. My working colour space is Adobe RGB 1998 and Color Management is set to 'RGB Preserve Embedded Profiles'. In CS4 I only changed the mode from 16 to 8 bits and then saved the file using a modified name. That was the only processing done in CS4. Both files (8 bit and 16 bit) were then re-imported back into LR4 and the histograms compared with the original modified master file. In both cases they showed highlight clipping indicating that the image is brighter than when starting the final processing in CS4. In most cases the extra brightness is not a big problem; but in some instances it is quite unaccepable and I could not use that version without further processing in LR4. Mode change does not seem to be the culprit.
Can anyone help me to understand what I might be doing wrong to bring about these unwanted involuntary changes to normally processed files? Thank you.

Answer 1

What colorspace do you have configured in LR for your TIF Edit-In export to PS?  Since PS is out-of-date with respect to LR, presumably you are using the Edit with LR Adjustments option not the Open Anyway option.
What happens if you don’t change to 8-bits in PS? 
What happens if you merely export the TIF from LR and re-import it?  With variations being:  if the Export bit depth is 8 bits vs 16 bits and if the Export colorspace is ProPhotoRGB vs AdobeRGB?
Do you somehow have lens-profile corrections set as the default for TIF Imports or any non-zero Toning being applied on Import?  The LR defaults for TIFs can be set independently from the defaults for raws.
What happens if you use ProPhotoRGB in your Edit-In export settings and as your working space in PS? 
What happens to the histogram if you soft-proof to Adobe RGB in LR?
Is your monitor calibrated with a hardware calibrator?  If so is the profile ICC V2 or ICC V4?