As complicated as colour management is it can be greatly simplified by the following understanding:
Assigning colour profiles preserves the data but changes the colour association. Converting to a profile preserves the colour but rewrites the file data.
If you want to preserve the colours you are seeing on the screen convert don’t assign.
I am guessing your mode conversion to greyscale was either from Prophoto ( gamma 1.8) or into greyscale 1.8 ( check your colour settings in Photoshop) and you assigned 2.2 colours to 1.8 data.
Hope this helps