While we are not sure what is causing this issue on your device, you could try the following fixes:
- Try making minor edits to the clips within the Video Merger (by tapping on the clip and choosing the “pencil” icon), this causes the videos to re-encode and may fix the issues

- 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.
- Try deleting the app and reinstalling it.

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