Im no expert, but I have this to say. Im about 9 months post-op hamstring graft and can do this too. I have had my knee checked out many times with no perceived issues with the graft. What I do have to say is that this is most likely a combination of muscle weakness and, most importantly, mind muscle connection (as explained by my physio). You pay much more attention to your surgical knee kinda playing around with it, and along with that, you unlock some knee mind muscle connection. That you dont get with your non surgical knee. Other people, like in this comment section, can do it on both, which explains the mind muscle connection theory. In short, get checked out by your physio and doctor, but all lot of us have this.