Maybe if I tell you what I've been doing it might help recreate the problem.
I had to replace the animation's frames with better ones, so I deleted all the frames except the one with the most image points that I needed, so I did that and added the new frames, I then applied the maximum number of image points that I had on the first frame to the whole animation (so I didn't have to go to each of the new frames and individually add the image points I wanted and I used the button that allows you to automatically add that), then I moved to frame 1 and deleted some image points. I did the same with frame 2, then on frame 3 I realised I needed a bit more so I added additional image points to that frame, and that is where it stopped working, I only needed a few more image points for it, and for or some reason the rest of the animation is able to have more than 21 image points.