If you don't train properly and open your body properly with the right chi kung, then attempt to hold the stance or copy the form like your teacher, your hip will usually be at the wrong angle, you will lean your back to compensate and this will prevent you from sinking properly. 9 times out of 10 this puts added pressure on the knees, which become misaligned to compensate.
When you rotate your dantian into the proper position, the whole body relaxes and sinks and there isn't any pressure on the knees.
So it surprises me that you would have problems with Chen style and not with other styles. Usually I hear people from various styles complain that it is Yang style which hurts the knees. Wang Xiangzhai noted the same thing about the legs in Yang style in his essay on Tai Chi.
It surprises you because you don't understand what is going on. Nothing against you, just I never gave a full explanation.
It is not lack of training properly, it is not wrong angle, it is not rotation of the dantian, itis not leaning, it is lack of cartilage in a knee with arthritis under the knee cap. And it is not holding a stance, it is shifting weight slowly between stances. A static stance is no problem at all.
Basically this stance, slowly shifting from left to right
Same training for this stance, left to right
An old knee missing cartilage with arthritis in it, doesn't like these....
No issue with Yang, Wu, Sun, Wing Chun or Xingyiquan. My knees just don't like low, moving, Chen stances..... ok, 1 knee, the one I was born with, the fake one had no problem with it at all.
However that knee, the real one, also had a meniscus repair a few years back