The sort field applied to the opportunity list in the details panel is overridden when you open the Forecast Expanded View and select a different field to sort on. Once the new sort field is selected, it will also apply to the list in the details view, but this is not immediately obvious, as we cannot display any additional columns in that view.
To restore the expected sort functionality in the opportunity list, please apply a sort on the amount field used in the Forecast column in the Forecast Expanded View