I don't have anything certain to tell you, but I'm in a similar situation to you right now. I'm 2.5 years our from an ACI on my MFC and two places on my patella. I had worsening symptoms over a few months (pain, catching, etc) and had a recent MRI. My surgeon tells me that there is fluid under part of my MFC graft and a "flap" or tear in the graft. I discussed the options and outlook with my surgeon's research coordinator. He told me that they'll do a scope and see how thick the tear is. If the graft has separated from the bone or the remaining cartilage is too thin, I'll have to have the ACI done in that area (or get an osteochondral allorgraft). If there is enough cartilage left at the site, they might be able to just leave it be.
Do you know how much of the graft was torn? My surgeon's research coordinator pretty much told me that any tear in the graft is bad news, especially with catching and mechanical symptoms. He thinks there's at least a 50% chance that I'll have to have another graft.
I know that's not much help, but I thought hearing what someone in a similar situation was told might help. I hope your knee doesn't need more invasive surgery!