So, I've encountered an issue sometimes for a few years already, and it's bugging me out right now. This usually happens when I duplicate an animation, but want to remove keys that are at the end of the timeline, for a single frame pose. The timeline length remains, and I cannot figure out the reason why. This likely happens when my animation is full of many elements: Bone keys, Attachment keys, Constraint keys, Active Skins. My temporary solution is to create a brand new animation and copy the first frame's keys, but this can be difficult with complex Spine projects. For example, I cannot select an attachment unless I select the Skin it is under.
I also have to point out that it can be difficult to reveal every keys in the timeline if the Spine model gets complicated. I really want to be able to reveal every single key, so it is easy to purge or copy a keyframe.