Hi,
The iMovie app on my computer does not reset the zoom level of the timeline. I am not aware of any default setting for the timeline zoom feature. The zoom setting remains at the same setting as when I last opened iMovie. Even if it did change, it would take only an instant to adjust the slider to the previous setting.
Since the zoom slider resets on your system perhaps it is a preferences problem. 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 empty library. Reopen your old library to get back to your projects. (To reopen your old library close iMovie and go to your Movie folder or wherever you have your old iMovie library stored. Double click on the library’s icon.
iMovie will open in that library.) Possibly you may need to reboot to set the changes. Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.
You could also transfer your project media to another newly created project and see if that cures the issue. Do an Edit/Select All, Edit/Copy, in your current project and then Edit/Paste into the timeline of the newly created proejct. Your edits will carry over.
-- Rich