I’m sure you mean when you type cmd+j to change the speed.
In the window that opens, there is an option to ripple the sequence. If it’s on, and you change the speed to say 50%, the length of the new clip will be twice as long as it was before. If it’s off, you’ll see the behavior you’re reporting. So it’s a setting, and actually a feature, because you can have the behavior be either way.
Jerry
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