IMO they should have a (second) pop-up that warns you that you only get one submit. Not sure if it should let you know if you've made an optimal solution or not, but since it's not timed there's no cost in slowing people down. I've seen similar daily puzzles where you get to see the leaderboard and then can go back and optimize further. Yes, it says it at the beginning, but it's still easy to forget.