Starting from September 2020, eBay provides a unique code called eVTN. It's automatically added to orders shipped via Royal Mail and Australia Post.
The eVTN code looks like ebay1234abc which will appear on the second line of the address.
Now M2E Pro imports eBay orders which include eVTN code by default.
If you don't want eVTN to be added to a shipping address line in your Magento orders, you can use Skip eVTN option in your Account Settings under Invoices&Shipments tab.