This is a consumer forum of iMovie users like yourself. We have no special incite, input, or control over Apple's software designs or decisions. So I have no idea what Apple may or may not be doing regarding iMovie or Monterey. Sometimes issues can arise with updates and upgrades.
Make sure that your iMovie 10 version is updated to most current version.
If all O.K. there, 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. Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.
If still no luck, try these diagnostic procedures to see if one or move of them cure the issues:
Opening iMovie in a new library
Try opening iMovie in a new library. To do that, open iMovie while holding down the Option key and then selecting to create a new library in the box that appears.
Opening iMovie in a new user account
Create a new user account from System Preferences/Users and Groups and try to open and use iMovie in the new account.
You can use the Guest account on your Mac for this purpose.
New Project
Another thing you can try is to copy your media into a newly created project and see if that cures the problem. To do
that, do an Edit/Select All, Edit/Copy, of the media in your project and then Edit/Paste it into the newly created project.
Safe Mode
You could also try booting up in Safe Mode (see Finder menu for instructions) and, after everything loads, immediately shut down and reboot in normal mode. Then see if iMovie works properly.
Updating iMovie
To update/redownload iMovie 10, drag your iMovie 10 app from the Applications folder into the trash, but do not empty the
trash. Sign in to the online app store and click on your identity at the bottom of the app store sidebar on the left. You will see your purchases displayed, including iMovie 10. Redownload/update iMovie from there. If not successful, drag your present iMovie app out of the trash (if you moved it there) and return it to the Applications folder.
Resetting the System Management Controller
Try resetting the System Management Controller (but no longer available for the new M1 computers):
https://support.apple.com/en-us/HT201295
Resetting the NVRAM or PRAM
Try resetting the NVRAM or PRAM:
https://support.apple.com/en-us/HT204063
Contact Apple Support
If none of the above resolve your issue, contact Apple Support. The techs there have the capability of remotely accessing your computer that we cannot do here. To contact apple support click on the Get Support item in the upper right hand corner of this forum’s screen. Navigate to a place where you can enter your phone number.
-- Rich