Just completed a File Share migration to a SharePoint Online library using Microsoft's SharePoint Migration Tool. The document library generated ID numbers are 2,4,6,8,... only even numbers. Anyone know why? Is this a bug? All data was uploaded.
I tried simply migration from FileSystem to SPO and the outcome is the same. It looks like the migration tool doing something inside. Probably if you want to figure it out, you'll need contact Microsoft.