Depends on the size of the company and if you applied to a specific position or not.
If the company is a large company this can happen. Sometimes a totally different division runs through candidates. For medium and small companies this would be highly irregular since they have no need for the extra layers of bureaucracy.
Secondly if you applied to work at a company and not a specific position then you could end up in a general pool of vetted candidates that managers can pick from and skip steps in the process like directly interviewing you. This is more common with entry level positions where managers are not looking for a specific experience/skill set. Again if you did apply directly to a position and it was not an entry level position then it should raise a flag.
Your situation happened to me when I applied to a rather large corporation right out of college. My interview was even in a different state than where I would be working. I did not meet my supervising manager in person until my first day on the job (had only two phone call conversations before that). The humor of it is I did not find out what I would be working on until after I accepted the offer (I was rather naive). If I had known I likely would have not accepted, but it turned out that project was one of the best projects I ever worked on.
I got lucky, in these situations it is best to ask questions and see if you could talk to who would be your supervising manager before signing anything.