It’s where there is a fusion failure of the anterior acromial apophysis. In a case-report published on the internet, there was a successful solution via taking anti-inflammatory drugs, resting the shoulder, e.g. no swimming for 2 months and doing gentle stretch-band exercise.
We did all that and Madison hasn’t been swimming using her shoulder since 3 months and the problem still exists.
Further medical invention will have to take place.
Here is another interesting article on surgical intervention.
When I suggested to give up swimming instead, the look I got in return could have frozen an iceberg.