Alt+period currently preserves the current selection if all units are busy, which can mislead a player into thinking that there were in fact some idle units. Resetting the selection gives a clear signal that all units are busy.
Start a new match, make sure that Alt+period still selects all units; then assign tasks to all units, select a busy unit, and ensure that Alt+period deselects it; finally make sure that Shift + Alt + Period still works (currently selected units are not deselected, even if they are busy).
To be honest, I find this behavior to be more unintuitive. Currently I won't lose my selection if I just wanted to check if there are any idle units (usually, if there are none I would do something with the current selection). But now, I would have to add the units to the selection after.
Maybe it's just me, but I think it would be best to remove that todo. Engine.PlayUISound(filename, looping); if interested.