raid
Dad Photographer
raid, are you shooting straight to jpeg, and then editing that? I haven't seen posterization in my images for years - the RAW/DNG files don't have any of that with proper editing. I never shoot straight to jpeg anymore, but I did back 5 years ago or so and ran into this problem occasionally, especially in challenging light conditions.
Also, perhaps a setting was changed when you sent your camera in for service that changed the bit depth for RAW files? I can't remember if that's possible in the M9 menu right at the moment and my M9 is at home.
I shoot always DNG files. Then I use LR5 or CS2 to get the jpg files.I just checked, and the M9 is set to uncompressed DNG.