I feel it is loosened during the upset process. And does come off before the strike or be fragment can be struck on the next coin. If it fell off before the strike, the coin would be under weight and have a weak strike:
If it were struck through a fragment and it fell off after the strike it might look like this:
The weight would be normal as the fragment fell off the coin.
If it split off after the strike, It might look like this: