Welcome to the very unpredictable world of Shape Tweening in Animate! For decades, the shape tweening tools has been very unpredicatble and unfortunately, very ignored. It has never received an update or an improvement. It's finicky and the simpler the vectors, the better chance of success. But I've seen epic failures with just 2 point segments.
In your case, you may need to simply address this animation in a different way. Hard to suggest a technique without seeing your intended animation/motion. Do you want it to simply retain the design as you have it in frame 125 of your document?
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