Once more:
The bag is closed during play, as of the "do not change clubs" rule of golf. Here, it includes the ball pocket - simple programming.
Think of you inventory to be in your garage - inaccessible during play.
Follow the indication of "last ball" and buy a new sleeve in time. Caveat: It doesn't work if your balls are no longer accessible in the Pro Shop.
If a player doesn't care for the balls, the round must be less important - he should simply quit.