Motion Keyframes Disappear

I'm having a recurring issue in which I add motion keyframes to a video or image layer, and it works fine upon creation.  But then I go back to work on other parts of the project and eventually return, the keyframes are "undone" - some of them just aren't there - so the animation/motion is lost.

In this example screenshot, the lower clip "MuseScore Mine Motif 1" has the desired motion keyframes.  The image "Endless Mine Microthumbnail" previously had the same type of motion, but you can see, the first "starting point" keyframe is missing, so the animation/motion is lost.  This problem has ocurred multiple times recently - does anyone know what the source of the issue is, or how to fix it? I have created several projects in the past using this type of animation/motion keyframes system and never had this issue, but lately it's been occurring frequently.