When you have a database setting to embedded images they are not migrating properly.
Database settings : Display images after loading is checked. During migration you receive the following warning. “An image was not extracted correctly. This may indicate a corrupted Notes document or an image that was saved in a way that we could not extract. Please try unchecking the "Display images after loading" database property, editing and resaving the document.”
Another error message for this issue is "buffer cannot be null"
Uncheck the box to display images after loading after in database properties. Edit and save document and execute migration.
If there are a lot of images to modify, you may want to consider creating a Notes Agent to process these documents. Support is unable to assist in creating this agent due to the complexity of environments and risks envolved.