How to reproduce:
1. Select an item in the DropDownList using mouse.
2. Press tab to go to the next field. Focus should go to the next field. Instead focus goes back to the first field.
I modified REPL from your demos, adding textboxes to reproduce the issue.
modified REPL: https://blazorrepl.telerik.com/cHawYHPd47d0122o27
modified REPL: https://blazorrepl.telerik.com/QxYmknPR49ZdbVQQ02
I fix this focus issue using OnClose event, setting the focus to the current DropDownList with FocusAsync(). But I have to modify all the DropDownList in my projects.