When Kirito enters Alfheim Online and notices his stats are from SAO, he quickly scrolls through the list of bugged items looking for Yui's Heart. He found it but it was renamed to something else, and when he used it, Yui appeared.
Yui's Heart was a very unique item in SAO as it was created from what Kirito could save of Yui after she was deleted. Since REC Progress made Alfheim Online based off of SAO, it would explain how all of Kirito's SAO items were bugged but Yui's heart remained and was renamed.
Why was Yui's Heart, the most unique item in SAO, the only SAO item in Kirito's inventory which wasn't bugged?