So, I just noticed, I'm using a macbook but using a bluetooth mouse (MX Master 3)... If you use the trackpad natively, you can enable the stretching the length of a layer.. but if you're using a bluetooth mouse, it will show 2 out of 10 chance only.
Sorry for the problem and for not answering you earlier.
I adjusted the video length with the native trackpad on a MacBook and with a Bluetooth mouse. Did you try to disconnect and reconnect your mouse to your computer? We suggest closing the project and trying to edit the length again.