I got a 'new' thing in a case I just finished. Until now I never experienced that one of the people I questioned accuse another person who even could not have done the murder. (Real alibi.)
Is this a common thing in the game and did I by accident never have this before or is it a bug or something?
Yes, this is normal. Some of your witness evidence (WE) will point to people with real alibis, just as some of your physical evidence (PE) will point to people with real alibis.