It's because it ran out of hits (life)
Below the golf ball on lower right portion of the play window where you add spin there is a hits counter. Each purchased golf ball has a durability limit and once you have used the ball up it gets taken out of play and a new one comes out of your bag. If it is the last of that kind, you will play the rest of the round with the WGT free ball.
I see you are using a callaway ball........good job on using the whole life of the ball, it is too easy to lose one before you run out of hits.