I ran into this same issue. Here are some vis that illustrate the point. It took me forever to figure out that the Tip Strip property was causing this problem!
Run Subpanel_outside and try to enter "12" in the numeric control. It will get the 1, then lose focus.
If you edit the block diagram for Subpanel_inside, switch the Diagram Disable structure to the other case which has no tip strip property and run it again. This time, it will not lose focus.