We were in Northern Michigan with the Prius for a few days this week at the Fireside Inn on Grand Lake in Presque Isle. (It's an old school Northern Michigan American plan resort.) We explored the Besser Natural area a bit and had to turn around in a narrow spot. The back end sunk into sand and started scraping rocks. LOL That won't be happening again, as I was working on a deal while there to trade in a couple non-Prius vehicles for a new Ridgeline. They (and my '85 Deere 318) look happy together in the garage.
That seems strange since there is so much less weight on the rear wheels - and they're not driving... I'd expect the front wheels do dig in first. On vacation, were you heavily loaded?
We weren't heavily loaded, but we were backing into a clearing to turn around. The front wheels (thankfully) stayed on solid ground.
Well, since a Prius is only 400 lb. lighter than a Wrangler, I guess I will take your experience as a warning!