Maybe it's not working for the english speakers community, because I see some people just leave asap in the end of the run without saying anything. But in the japanese community, they usually say 'otsukaresama' or 'pt arigatou' in the end of the run regardless of how bad the party performs, maybe SE gives out the commendation on top of it to reward someone who perform the best.

I agree that it feels kinda broken when someone already left, we cannot commend that person. It kinda force everyone to stay in the dungeon for a bit.