I have hesitated to post this example because I don't think it's really a "notable mathematical development" as such, but after seeing the other answers, I think this one is worth mentioning.

As reported in Scientific American, Epoch AI invited several mathematicians, including Ken Ono, to a meeting designed to generate challenge problems for "FrontierMath". Among other things, Ono came up with what he thought was a Ph.D.-thesis-level problem: "What is the 5th power moment of Tamagawa numbers of elliptic curves over $\mathbb{Q}$?" To Ono's amazement, the AI autonomously solved the problem. You can read Ono's account on his Facebook page (also reproduced below), or listen to him talk about it here.

Even if this is a cherry-picked example—the best one from the whole meeting—this strikes me as a very impressive achievement. But see also this tweet by Daniel Litt, who was also one of the invited mathematicians but was not too impressed when he read over the chat log.