In "Source Data Definition", if "Convert Windows Metafile Image" is checked, EMFs in Notes Documents are migrated to SharePoint list as specified raster format(JPEG/PNG/GIF/BMP).
But some images are not migrated correctly when migrating to SharePoint. Images become distorted or corrupted.
This is a known limitation.
When Converting EMF Images to other type of image, such as PNG or JPEG, those may lose the quality of the images.
please also refer to here.
Migrate affected Notes documents as document files in the target SharePoint's document library instead of list items.
If the target template is document, such as MS Word or PDF, Windows Metafile Image (EMF) can be supported well, and it doesn't need to convert to other type of image.