Hi,
Modal, by definition, keeps the front panel on top of all other LabVIEW windows until you close the window or open another modal window. Note that you cannot interact with other windows while a modal window is open. It's just like option dialog boxes in any other program.
One way to get around this is to have the SubVI "show front panel when called". To do this, simply right-click the subVI and click "SubVI Node Setup" >> show front panel when called.
I hope this helps!
Regards,
Nadim
Applications Engineering
National Instruments