Uploading new image to existing imagedoc changes fields in other documents

Reproduction steps:

1. create project based on archetype
2. add the simple content feature from essentials (rebuild not necessary)
3. in the CMS document type editor, add a Boolean field to the type
4. save the type (type actions / commit)
5. create a new document from the new type, check the boolean field, click Save & close. Leave the tab there (don't click the red X-icon on it).
6. add a new image in the images section, after uploading the image the image set document is shown in edit mode
7. Edit the image set document and upload a new original image, don't save the image set document
8. Click on the tab of the simple document: the checkbox of the boolean field has disappeared!
9. Inspect the nodes of the simple document in the console: the boolean property is set to true on both nodes.
10. Log out from the cms: the session of the editor is saved and the boolean property is set to false on the preview version node.

This changes the content of documents in a most curious and unexpected way and should be fixed as soon as we can.