Wordpress: 5.8.2
ACF: 5.11.4
Re-produce:
1. Create a new custom field
2. Add a new field as group
3. Add a new field inside that group field as group
4. Add a few new fields as image and choose URL
5. Save
6. Open nested groups and you will see that some of the images are set as Object
7. Also those object image fields will return image ID when you call get_field.