I am not a coach but I would gladly express my non-expert opinion, if your permit.
I second everything that has been written hereabove.
Also, I think that your case is typical of swimmers with "hammer" feet. Instead of kicking with feet straight (in-line with the leg), the feet are at a 90° angle with respect to the leg. The feet push water forward, thus propulsing the body backwards. Some swimmers actually move backwards when kicking (no arms)!
This phenomenon is enhanced when the hips is low, on the down kick by pushing water forward with more surface area (the foot and the lower leg), and on the up kick by overly bending the knee. A bigger kick amplitude also enhances this phenomenon.
When flutter kicking, the feet interact less as they are "dangling" behind the hips.
It (hammer feet) can be seen in the video of yourself that you posted last year, where you flutter kick: