After Amazon implemented the changes for the inventory data import, its API might not return the Unmanaged Item titles. You can notice that the Unmanaged Listings contain '--' instead of Item titles. The issue does not affect Amazon Items that are already placed and managed in M2E Pro Listings.
To overcome the issue, please upgrade your M2E Pro to the latest version. It may help retrieve the Amazon Unmanaged Listing titles within 24 hours. If the Unmanaged Listing Titles remain unrecovered, there is no need to worry. You can still link these Items to Magento Products by Product SKU or ASIN/ISBN, then move them to M2E Pro Listing.