same happened to me.
https://mtga.untapped.gg/replay/Um9eT26Leo4aC7bRzbudwD
apparently need to cast a certain amount of spells. If that's the case, please add it to the achievement description

Same thing here, I completed the your go achievement but it didn't registered.

I completed my 16th midweek magic event but my achievement remains at 15

I have collected 4x of every card in a set, it is not registering for the achievement.

Really, u need 5 spells !!! it s so stupid, the games are so quick!!!

Edit : U right, 5 spells mini

Same problem here, i built a deck with only flash cards i played them only during opponent's turn, i won and the achievement wasnt completed!!!

Also encountered the problem. I had built a deck specifically to get that achievement, and was successful twice, but neither counted. The deck is R/G and Explorer legal, using Smuggler's Surprise to put Ghalta and Terror of the Peaks into play on my opponent's turn, then winning either through burn by putting more creatures on the field via Ghalta, or simply running the opponent over next turn. Did both, still no achievement. I did nothing during my turn, except surveil 1 via lands. Does it take a certain amount of actual spells? Because I only needed 2 spells on the first attempt, then only 3 on the second. The rest was activated abilities.

EDIT:
Alright, I searched the internet a bit, and it seems, that you have to cast at least 5 spells for the game to count and to get the achievement. Really annoying, but it worked out (even though I had to stall the game and change the deck a bit to get UP TO 5 spells before winning).

Same. Are there undocumented requirements for this achievement—a certain number of turns, winning by damage and not timeout or resignation? I’ve won at least twice playing nothing on my own turn but lands, and no achievement.

I don't know if there is a fix by now but I've completed it multiple times today and still haven't received it.

same problem: https://youtu.be/dIybxochLpk