I have no official source but know that this happens a lot. Also the arguments with the employer about the letters afterwards. Some are so fed up and let you write the first or final draft. There is also the hidden code. So instead of writing something negative which is forbidden you just use different words or leave out some intensifications. Like “zur größten Zufriedenheit” vs “zur allergrößten Zufriedenheit”. One means your work was Ok the other it was great. There is also intensification by adding time adjectives like “always” or “often” etc.

This code is known by people in the HR and hiring departments. It’s a very weird praxis. I have to explain this to my non German colleagues because for them even a mark F letter sounds awesome ;)

My question would be: why even bother with any kind of code? What incentive is there for the employer to write anything truthful, to write anything but the blandest most positive things that really don't say anything hidden?