I'm confused about what happens to the KFs you've set in an image object (usually a photo) when you change its length.
Take a solid example. Place a photo of say 10 secs on the timeline of a fresh project. In Size/Position, add a KF at the start, zoom size 100, and one at the end, size 200. Ensure that Combine effects curve with object length is not enabled. Now use Right click > Change photo length to shorten it to 8 s.


BTW, if this was a built in effect, such as a movement template, you'd have great difficulty adjusting it. Because you couldn't access all its KFs without removing it to some track (or another movie) where you could expand it to its original design size to see all of them.
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Terry, East Grinstead, UK