Using "du" instead of "Sie" for adressing people can be a very serious issue. If you do this to a policeman, for example, you can be fined by typically 600 € in Germany (source: https://www.bussgeldkatalog.org/beamtenbeleidigung/). To give an analogue in the English language: you wouldn't address your boss with "yo nigga". So I believe the reprimand was rather rightfully.
A rule in the German language is: in doubt, use the more formal language register. The "default language register" in German is typically "one register more formal" than the "default language register" in English.