Hello. I had a TKR nearly four months ago and am experiencing a very similar issue and believe it might be related to a tight IT band. I found some helpful information on the American Association of Hip and Knee Surgeons website --
https://hipknee.aahks.org/wp-content/uploads/2021/01/IT-Band-Handout-knee-Jan-2021d.pdf.
According to their site, "The IT band can become irritated during motion of the knee as it rubs back and forth over the end of the thigh bone or top part of your knee replacement. Continued irritation can lead to inflammation causing pain, tenderness, and swelling over the outside portion of your knee." I also have knock knees which can exacerbate this issue.

Although I have very good range of motion and flexibility, I have pain on the outside portion of my knee with a loud popping sound when I bend, flex or go down stairs. Not so much going up. I'm continuing to ice, do lymphatic drainage massage for inflammation and swelling and have started to wear a neoprene compression sleeve. I am also restarting PT to try to help loosen the IT band and break down the scar tissue which I think is exacerbating the issue. Hope this helps you and that you can get to the root of your problem soon.