I'd have to guess (and this is definately a guess) software update:
1st Service applies update which changes 'box parameters. You don't like it and dealership resets the settings back to where they were (possibly even reverts to previous software??).
2nd Service finds no applicable update (or main dealer remembers that you don't want it, as it's in their records) and things remain as they were.
3rd it gets an update (or specialist notices that original update has not been applied) and you're back to the new M-B default settings again.
That would explain the lack of reported faults, as there wouldn't be any. Ask your specialist if any routine software updates were applied as part of the service....
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