The contributors to this forum are consumers like yourself. We have no inside knowledge or input into Apple's decisions or plans regarding its software.
Meanwhile, you might try deleting preferences, as sometimes that cures export issues. 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.
-- Rich