Just as we were finishing setting up our new 29gal tank about a month ago, Spider decided it was a perfect time to molt one last time in his old tank. So for the past month, the others have lived in the new spacious tank while Spider has been in the 10gal molting (aka we've had a glorified sandbox in our bedroom). This morning he finally was up, perched on the edge of the salt water dish! Hairy legs, sharp black toenails, the whole nine yards. Excited to finally have all my crabs in one place (and to finally be able to decommission the 10gal (unless we end up needing an iso someday), I moved him into the new tank with the others (after a quick dip in the freshwater to make him smell more like home and less like molt). I was late for work at that point, so off I went.
Came home from work, and Spider was happy as can be in the new tank... but on closer inspection, he seems to have come back from his molt missing a leg! One of the middle ones (the longest leg that is closest to the shell opening). Before his molt, none were missing. I'm not positive he was missing it this morning, but I don't see any evidence that it came off today while I was gone-- the other crabs who are above ground haven't strayed from their coconut hut all day, so no one's been picking on him.
Was he actually not done with his molt, and I interrupted him by moving him? Should I put him back in the old tank? Should I leave him in the new tank? New tank has better heating, better humitity, and more sub, but the old tank has the benefit of privacy (and whatever molting cave he was using, if he wants to go back to it).
Any advice is appreciated

I'm posting this here rather than in the emergency section because he doesn't seem bothered by his missing leg... but I'd like to do what's best for him!