That's a tough spot for those horses. Seen it myself when folks get a bit too keen with the hoof knife. @leob97 has the right idea about leaving that safety margin. It's not just about the trim, it's about letting the foot build its own defense over time. Taking off too much just leaves them tender and at risk for bruising. A good farrier can teach you to read the sole and know when to stop.
See this a lot. People get eager with the rasp and take off too much sole. The hoof can't protect itself, leads to soreness on hard ground. Best to leave a bit of sole, let it callous up. Have a farrier check the trim every few weeks. They can show you how much is enough.
Ever seen a horse go lame from being trimmed too close? @wendyj24 is right about leaving that bit of sole, and I always leave about a thumb's width for safety. You can actually feel when the sole gets hard enough to stop, which takes the guesswork out.