Hi, I'm Jina! You're right. 안녕하세요 (An-nyeong-ha-se-yo) is Hello as you said, and 안녕히가세요 (An-nyeong-hee-ga-se-yo) is Bye. It's common that Korean doesn't really pronounce them clearly. Sometimes you can only hear 하세요 (ha-se-yo) because they barely pronounce 안녕 (an-nyeong). So it's possible that they just didn't pronounce 안녕히가세요 (An-nyeong-hee-ga-se-yo) clearly as well. Even when I say 안녕히가세요 (An-nyeong-hee-ga-se-yo) fast, it sounds like [An-nyeo-e-ah-se-yo], almost like [An-nyeo-ah-se-yo]. You might be confused because of that and think that was 안녕하세요 (An-nyeong-ha-se-yo). I hope it will help you (:

안녕하세요 literally means "Are you at peace?" 안녕 is interchangeable because it literally means just "peace", but if you want to be formal: 안녕히 가세요 -Literal meaning: Go in peace. You would say this if you are staying and the other person is leaving. 안녕히 계세요 -Literal meaning: Stay in peace. You would say this if you are leaving and the other person is staying. Also, if you speak chinese, 안녕 was borrowed from 安寧. As you can see, they sound very alike. Answer by: Miapapayapia