What iMovie version and operating system version are you using on your Mac? What is the format of the portrait photos? HEIF? HDR?
Meanwhile, try deleting/resetting preferences and see if that cures it. To delete/reset 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 empty library. Reopen your old library to get back to your projects. 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.
Also, create a new project and add your portrait photos into it and see if they display properly there.
-- Rich