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Tight glute on one side or something? I'd be getting a physio with bike knowledge (velophysio) to look at it and give some remedial exercises, stretches.
Also check it's not something daft like your saddle being misaligned, sagging, or one of the rails bent (I had this once and for ages couldn't work out why I was getting knee pain on one side until cleaning the bike one time I spotted the saddle on an angle).
Anyone one else have a twisty pelvis? As I get older I seem to be twisting more to my right. At the end of a long ride my saddle is pointing 2-3 degrees off to the RHS.
This is causing some knee pain now as my left leg is trying to extend further forward relative to my right … Any way to counter act this?