Just to clarify something, the "Cornering the Stalk Market" achievement isn't new, that achievement has been in the game since the very beginning.
Is it possible that you bought turnips but let them spoil, either through TTing or forgetting to sell them? Since the achievement itself is for profitting on the turnips, rather than outright selling them, and the game does keep track of how much you've spent on turnips internally. So if you bought 10k worth of turnips and sold them for exactly 10k, you'd still be at 0 in terms of profit. If you bought 10k worth of turnips but didn't sell them, you'd be at -10k for profit, and so on.