[Bayart]

>I have a close friend that works at the regional Lidl HQ and she got reprimanded just after being hired for having used the singular "you" (less formal) instead of the plural "you"

Sounds reasonable. If someone did the same to me in a professional setting my instinct would be to treat it as an intentional lack of respect. Albeit my language is French (but it functions the same as German in that case).

[smnrchrds]

As someone who is familiar with formal/informal you difference in 3 languages (French, Farsi, and Azerbaijani), I had the same reaction. But I guess things are difference in German.