No responses, so I will take a shot at your question, not having experienced it in an R21 myself. (Probably no one wants to fess up to really knowing!)
I would think a "soft" grounding or one due to a tidal drop would be no problem for an R21. Obviously if the prop or rudder gear hit something hard in the process, damage might be caused. Or if the boat settled on a single small rock which tried to take the full load as it settled you might get some hull damage. But that would be true in most modern designs.
I am pretty sure that unless the boat were held across a tidal rip as it refloated, or faced some other very severe and unusual conditions, the boat would just rise as the water-level did and you would be back in business. The boat shouldn't lie so far over on its side that the water would overflow into the boat before it floated.
One thing to always keep in mind if grounded accidentally: Get out into the water and scrub the bottom, looking like it is exactly what you intended and that it was entirely your choice. 😉
Been there and done that.