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Interesting. Sorry, i keyed in on the knee rather than the sitting pain. That honestly sounds more like a sciatic nerve irritation. Just a guess, having seen a couple people have similar issues/pain (sans knee “pop”). The “pop” may have been unrelated or just due to general tightness or the quad/ham and the patella glided back into proper position? Speculating of course, but i’ve had the same sensation just without the leg pain.
I’d still advise working on hip abduction strength, and as you already were doing…loosening hams and quad (and calf/ankle/foot).
Good luck. i hope it doesn’t occur again!