No screenshots becuase it happened to my daughter, not to me.
She challenged me to a game of Tavern Brawl, since she still had to win a game there, and she had the 80g quest. We played, I let her win, I got 80g, she got 80g and a pack for the TB … but to her surprise, she also received a completion message for the legendary “play 1 arena game” quest, and got the 200 gold added to her account.
No complaints, of course, but it’s still wrong…