Well yes, but that´s not the solution I am looking for. The request is to have the same picture style for both the default login screen and the login screen that is displayed at shared links.
I´m running a different Nextcloud server with version 19.0.5 and there it works fine as expected all the time since the instance was created.
But nevertheless, the problem seems to be present wether the NC instance was upgraded from v19 to v20 or creating a totally new instance of v20.
As you proposed to use the app “unsplash” is not an option in my case. I´m dealing with shared links for customers and they should see a custom picture at the login screen but it should not appear in a tiled view.