In my experience, it is a common problem for taiji beginners to feel strain in the knee because the rules of Taijiquan say we should “rotate our hips”. When the kua (the technical term for the hip area) is not open enough, the knees tend to overcompensate and end up rotating. But the knees can only bend and unbend, they aren’t meant to rotate, so when they rotate, it leads to injury.
This can be prevented by paying attention to alignment. One simple way doing this is to keep the knee on top of the foot. Strain comes when the knee goes too far either on one side or on the other, or if it goes forward past the toes.
At a beginner level, the issue is potential injury. At a more advanced level, the issue becomes an inability to generate power