I saw some obviously well-travelled systems that didn't have the "discovered by" label and I don't think scanning them with surface scanner changed that. I think there are some systems that don't have label but also can't be your discoveries.
I don't think so.
It's either known (such as Sol) or unknown.
If it's unknown and someone has scanned it and sold the data, their name will be on it. If it's unknown and no one has sold the data, it's up for grabs.
You can see several systems inside civilised space with commander names on. There's no such thing as an unknown system which you cannot get first discovery by scanning it first and selling the data.
If you can scan it to get data, you can get first tag. Obviously, if you're first.
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I found a system unexplored except for the star by a beta backer without a proper name. I was surprised because its pretty local. Scanned it all, and sold it, yet it still has no discovered by name. I'm guessing bug.
36 Ursae Majoris
I've not noticed this. It's always worked as expected for me.
Maybe it's a bug. Did you actually scan the bodies that had no name attached? Or did you just disco horn and move on?