I'm wondering if you have some goobery-bit inside the knee joint that hasn't resorbed or dissolved or remodelled or something yet.

Sounds like you have all kinds of strength from all those one-legged squats and stuff, but it's the extension range that disappears after repeated limited range flex/ext of pedalling?

Do you want to try an experiment?

Take a "snapshot" of your knee: how it feels, how easily it bends and straightens, how it is to walk.

Then bend your knee by hugging it to your chest 10 times. (hands around your shin, hug) (like a gluteus stretch)

Do another "snapshot." Is the knee better, worse, or the same?

Then sit on the edge of a chair, put your heel on the ground, and push the knee straight with your hands on the lower thigh 10 times. (like a hamstring stretch)

Do another "snapshot." Is the knee better, worse, or the same?