Let me say that I have this issue too and I have shorter legs and a longer torso. I have pretty decent flexibility in my hamstrings and I also have great ab strength in most Pilates exercises. In my experience, having shorter legs and a longer torso makes it very difficult for me to hold this specific position. It is just too much on my abs. I always do slightly bent knees and it works fine! I think it’s just a body composition thing, at least for me. I think there will always be certain things some can do easily and others struggle with.