We know a few users are still seeing this bug, we are working on fixing it as quickly as possible. You could try some of these options to see if it fixes the issue. If it does, please let us know as it will help us fix the issue permanently:
Edit the individual clips with the video, make small edits and then export
Crop or trim the video in your Photos app before importing into the video
Try removing some clips and then exporting, then adding more clips to that export later.
If you made a purchase and would like a refund, refunds must be requested from Apple Support, as the transaction is between the customer and Apple. Apple does not allow app developers to issue direct refunds. Request a refund from Apple here: Request a refund for apps or content that you bought from Apple