People Names black Bar

Photos wants to align the eyes and fills it up in case the key photo doesn't deliver enough pixels on the bottom to do so. If you want to get rid of these bars, just choose another key photo.

Surely the bar is there when the faces are being updated. Come back after a little while and the bars are gone.

Have a look at this discussion: Re: Why do some of the key photos on the people page have a translucent grey bar across the bottom?

We have not yet found a conclusive theory, but it looks like the black bar will be showing, if the thumbnail has a very light color and the write letters would otherwise be hard to read. Or, if the face is close to the border of a photo and the thumbnail is only square and would not align properly (Rysz theory in the discussion I pointed to).

This is correct.

I thought that at first, but the faces scan has finished for my library on all devices synced with iCloud Photo Library and a few black bars are still remaining. These People albums no longer are showing "Updating". But they are only containing manually added faces, because Photos could not detect any of their faces automatically.

Sorry - not correct. Photos will adjust the position of the frame so that the eyes are at the correct level in the thumbnail.

The black bar simply means Photos hasn't finished updating that person - look back later and the black bar will be gone. It may take quite some time.