That is pretty slick how you captured your screen. I still don't know what is different? I see you aren't really searching AD so I modified my AD properties so that I wasn't really searching and still don't get the click on the button like you do.
I entered the first 3 letters of the name:
I clicked on a user and hit tab. The button is highlighted, but does not click.
I then clicked on the autocomplete textbox with the name again and hit enter. I then get a "Processing" message, but not sure what is being processed?
I changed the message for the button to show "Button Waiting" so I would know when the button was processing. If I click on the button or hit enter when it is highlighted, I see "Button Waiting".
The user is then added below Users Selected
Thank you for looking at this and giving me suggestions!