Premiere Pro Problem: No animation between keyframes when using Opacity Mask

I have been opacity masking a moving object in a video clip in Premiere Pro.  I have about 25 keyframes and Premiere normally automatically creates the frames between keyframes like it should. However, for some reason for this clip it no longer does it. Instead, it will hold the frame of last keyframe I've made until it reaches a next keyframe of mine and then updates to it, then waits again until it reaches the next keyframe I have created etc. So, the animation between keyframes is missing.

I have tried nesting and copying the clip, deleting the keyframes and pasting the same keyframes to the new copy of the clip.

The keyframes are functioning normally on other clips (Premiere creates the frames between keyframes and forms a smooth animation).

I use Windows 10.