This atlantis rises I’ve encountered this problem.
I went to do some hard levels, and I failed to do it. However, it marked those 5 coins as received, and as expected, I didn’t get anything since I lost the battle (I checked with that recent activity view in the game as well).
Happened twice already, first time I wasn’t sure, but now I paid attention.
Game build: 1512
So I cannot utilise this atlantis rises to conquer hard levels, since I risk losing the coins if I fail at first attempt.