I played through the game and after the last mission-block (Nocturne Op55N1) and the Credit-Screen I was sent back to Night City - but the quest still remains in my questlog - is this normal?
Also some items I found along the last mission are gone - directly after the Adam Smasher fight they are still there, but once I enter the Mikoshi and do all the stuff there, the next time i can enter my inventory my stuff is gone (except for one gun - seems kind of random) - this is a bug right?
No, it is not supposed to disappear. I put the explanation in spoiler tags, since this is about the end of the game:
You can have different endings, but they don't support a continued playthrough in the open world, because either the character dies, Johnny takes over the body or the character doesn't really work as a "contractor" anymore. So the game in its current state ends after that mission.
After the credits you'll get a notification thanking you for playing the game and saying that you can continue at your last open world save, so prior starting the mission with the "point of no return". This also means that save also contains the open "Nocturne Op55N1" mission. That is why you still have it in your journey.
You can see the popup here in the picture (source).
Regarding the inventory. I didn't had this issue on my playthrough, so I guess it is a bug and not intended.
