ProductMediaMapper (4.0.1) does not populate _images

[NilsN]

What does your configuration look like?

If you use DisplayImagesMappings it will map images to the _images property.
https://docs.litium.com/documentation/add-ons/product-management/product-media-mapper_1/configure

Media mapper - multiple images on one field

For the DisplayImagesMappings you don’t need to specify a field name, the system is always using the _images fields that is the field for display images, this field also supports multiple images. if you using the FieldFileMapping instead you can only map one image to the field.