iMovie moving clips and randomly deleting them

You could try booting up in Safe Mode and, after everything loads, immediately shut down and reboot in normal mode. Sometimes that cleans out glitches and conflicts. Don't try to start iMovie while in Safe Mode because it will not do so.

Start up your Mac in safe mode - Apple Support

Another thing you can try is to open iMovie in a new library. Open iMovie while holding down the Option key and choose a different library or create a new library, in the box that appears.

--- Rich

As you have found, iMovie is not operating properly.

Try deleting preferences. 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. Now see if iMovie functions correctly.  Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.

Another thing that you can try is to create a new project and see if iMovie operates correctly there.

-- Rich