Yes—this can happen if the strap is worn in a way that lets it slide off the shoulder once the camera is lifted. The community advice is to wear the BlackRapid so it goes across your body and over your head/torso, rather than sitting in a way that can simply slip off the shoulder. In that cross-body position, the strap has no easy way to fall away when the camera is raised.

So the fix is less about tightening the under-arm section and more about making sure the main strap is routed securely across your body as intended for a cross-body sling. If it is only resting on the shoulder, it can drop when the camera is no longer hanging from it.