When selecting a play area in the Step Program mode, the pads stop reacting. The manual says “When a selection exists, some functions change their behavior and additional functions are available”, so I assume this “bug” is covered by this explanation.
Well, I just wanted to loop the selected area and try to play on top of it… So it does look like a bug.
I tested this and I just recorded new notes on top, whether in MM or main slices/one shots.
Do you mean add in notes within the step program (I would not call that ‘play’ though) ?
Turn edit off and you can play over the running pattern.
Either way, I cannot duplicate the OP’s issue.
I think I get what you were trying to do… tell me if I’m wrong 
When you create a selection and then long press 5, the selection stays and the cursor is moving. It looks like you can play the pads like in pattern mode with rec on. But if you remove the selection, you see that you can only enter at the current cursor position. Right?
And if you only want to play and not record, all you need to do is remove the selection while the loop keeps at the same loop length.
Here’s a video S2400 Pads don’t play when area is selected - YouTube
That’s it, right?
Yes, you are right! I got confused with the Rec/Edit mode.