Roundtrip mismatch for 8-bit to 32bit to 8-bit images. Gamma appears to not be reversible.

I am trying to debug a slight difference in gamma correction from 8-bit to 32-bit VS  32-bit to 8-bit.
The observed issue can be seen in darker, shadowed areas of the image. To recreate, try the following:
1. Find a good test 8-bit image, open in photoshop
2. Create an empty file of the same resolution, bit depth - 32bit
3. Select all in the 8-bit image, copy.
4. Paste in the 32-bit image
The image should look a bit washed out (like a gamma 2.2 has been applied)
5. Selected the pasted layer, add an exposure adjustment layer. Set gamma to 0.454545 (eg. 1 /2.2)
6. Convert back to 8-bit. Select image mode 8-bit. Choose merge.
7. Under type select 'exposure and gamma'. Leave exposure @ 0 and gamma @ 1.0
This should look close to the original image.
8. Flip between the original and this round-tripped image -- they are slightly different, especially in the shadow areas.
A gamma correction should be reversible. Since we promote to float - there shouldn't really be any precision issues.
Any thoughts?
cl

Well, but do you actually use color management? Unless you work perfectly linearized and can be sure the source image doesn't already have built in specific color skew and/ or PS is applying a correction based on color profiles, your values will never match. also the conversion back from 32bit will never yield a 100% exact match due to simple quantization issues when going from float to integer values...
Mylenium

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