It's been a while since the ticket was created, yet I have tried reproducing the issue. The Insert module appears to work for me in combination with the IEF module. I have set up a content type that has an image field with the Insert module configured, as well as having a reference to another page/node that features an editor. Uploading an image, it is possible to insert the image into the editor field of the referenced node.
Relevant documentation on the Insert module is here: https://www.drupal.org/docs/contributed-modules/insert →
Please note that the Insert module does not work with CKEditor 5 at this point.If the ticket is still relevant, it would need more information on the actual setup of combining the Insert module with the IEF module in order to provide more specific support.