Well, I have updated and retested with exact same queries as you.
The list shows correctly and the user id corresponding to the username is correctly inserted in the input, but then after a save the field stays empty, nothing is registered in the DB
I have tried to unpublish several plugins like autoaction, conditional, core field ajax, privacy but I still get the same empty value stored.
Also tried without AdminTools activated but still the same.
Mind to have a look at our backend ?
I send the info via PM, field id is 424