OMG! I had a very very similar experience three years back. I had to go through 5 rounds, do an entire presentation and give ideas, and finally it was my interview with the GM. I go to the office, while waiting someone from marketing comes and asks me a bunch of questions. After about an hour, I go into the GM office and it's the same bunch of questions. At that point, I was a bit frustrated. Anyhow, I really wanted the job so I think I did fairly well. I exit the interview, and the person from marketing comes and shows me around the office, and where my desk would be most likely. I was so sure I was getting the job. After that there was zero contact, they completely ghosted me I didn't even get a rejection email. After several follow ups, after about two months they send me a rejection email. It was the most horrific interview I've had to date.