I cannot replicate what you are describing. When I attach an audio clip at a point in a video clip, then delete the first 10 seconds of the clip, the attached video remains attached to the video clip at the same point in the clip where I placed it. Can you post a couple of screen shots that show what you are seeing?
Since that is not happening for you, try deleting preferences and see if that cures it. It is a safe procedure that will not cause loss of data or disturbance to your project. To delete preferences open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects.
As a workaround, you can move the audio attachment point to anywhere you want in the video clip. While pressing down the Option and Command keys click at various places in the audio clip. The attachment handle will move to each place that you click.
-- Rich