Hi, LAinNC,
Possibly a corrupt preference is causing the problem. Try deleting preferences, as that often cures iMovie issues. Deleting preferences is a safe procedure that will not cause data loss or disruption 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. Now see if your brightness tool works properly.
If the above is unsuccessful, there could be another cause of the problem. If by chance your videos were recorded in iPhone 12 with the HDR setting, and your videos or photos look overly bright or over saturated, then that would be happening because iMovie for Mac does not yet support HDR from your iPhone 12. You would need to go to your phone settings and turn off HDR for recording future videos. Go into the camera settings and turn off HDR under the Record Video sub menu. Once the HDR setting is off any future recordings should work in iMovie for Mac.
If you can’t or don’t want to redo your HDR videos, on Mac open your video in QuickTime. Then export/save the video in H264 format. It will strip all HDR meta data so you will be able to use it in iMovie for Mac.
-- Rich