I've had toenails, but never a rear foot. To be honest I couldn't tell you, but I can't say I ever remember having lost a beaver with a solid rear foot catch. I am careful with trap placement when my water is shallow and so I virtually never rear foot them there. Only once have I came upon a live beaver in a trap, although twice I can recall doing so with otter. The beaver and one otter were back when I was a teenager and didn't carry a pistol on the line with me. The otter had the bank stake pulled and the drowning wire twisted so he couldn't go down it, so basically he was on an 8 foot leash staked out in belly button deep water that had a skim of ice on it, and I was wearing hip boots. Getting out there and dispatching him with the back of an axe was a real experience! Everytime I would swing he would dive, then come up on the other side of me.
Basically if I make sure I front foot them I know they are going to be dead and I don't have to worry about losing them or dealing with a live angry beaver when I get there. Besides, I was taught to front foot them when I was kid, so that's the way I've always done it.

And although I've caught quite a few peg leg beaver over the years, I've never pegged one that I recall. Only time I can recall coming and finding a foot in my trap was when it froze over the pond one time and there was just enough open water on the spillway for the beaver to stick his foot in, but not enough for him to go down the drowner. But he was still there when mister bobcat came along and decided he wanted a meal. Found the foot in the trap and the beaver about 20 feet away. You know that bobcat skinned that beaver about as slick as I would! He just opened him up from the side instead of the belly.
Sometimes thieves if I am along a road, wolves or possibly cats could all be an issue, but the main reason is because I want them dead when I get there.
330s are good and useful, but I set about 10 footholds for every 330, more versatile in my opinion. And some of the local bobcat trappers in the area trap a few beaver every year for bait, and they ALL use 330s, I don't think any of them know how to foothold beaver. Consequently you end up with 330 wise beaver. Whereas where I grew up the 330 was what you grabbed when you had a trapwise beaver, here I grab a #4.
And don't get me wrong, I've caught quite a lot of beaver by the back foot, if I have plenty of water and there is a better spot to bed the trap deep for a hind foot than there is shallow for a front, I'll set it deep. Also trapping rivers where the water fluctuates you'll catch by a hind a lot when the water comes up. Just a preference and the way I was taught to front foot them. I've had a hind foot beaver pull 50# of window weights up off a hard bottom and be sitting on the bank waiting for me, I've never had a front foot caught beaver pull a drowning stake or weights up.