Thanks for your reply, I was adding new free accounts to a Family account.
But I got stuck at the displayname, even after deleting and recreating the account.
However, I gave up and went on the PC just to change the avatar. And when I created the next account from the PC this time (instead of the Android app) the Name was displayed correctly in the PC app, and afterwards also in the Android App. Again deleting and re-creating the previous account from the PC worked and instead of the ramdomized User ID the Displayname is now seen where I would expect it.
(The user account from the original post no longer exists, hence I reply from my main account)