Stem doesn't currently support splits for composition earnings. We calculate the statutory mechanical royalty from US digital downloads & pay them to the content uploader on a monthly basis. For more context on this, please see Why can’t I enter composition splits?.
Unlike sound recording (master) royalties, composition royalty earnings are not visualized in the Stem dashboard. Any composition payment totals will appear in your PAYMENTS tab (details on each payment can be viewed by clicking into a specific payment).