In case it isn't an HDR issue, try deleting preferences and see if that cures it.
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.
You can also try using the free download, Handbrake, to re-encode the troublesome clips to H.264, Mp4/AAC with the free download, Handbrake. Do that even if the codec already is indicated to be Mp4/AAC. Re-rendering often cures issues.
You can get Handbrake here:
https://handbrake.fr/
A simple way to do it is to open Handbrake and do a File/Open Source. Navigate to your video and choose it as the source from the resulting screen. Then do File/Start Encoding. Wait a couple of minutes for the conversion to complete. Then save and import the converted clip into iMovie.
Try it with a duplicate of the clip so as to preserve your original.
-- Rich