Uploaded images are cleared in editable content properties when saving another property

Steps to reproduce:

1. Create an AWM app with a checkbox and a content field
2. Create a new entry in the app and save it (empty)
3. Click the edit button of the content property
4. Click the "image" button to insert an image
5. Go to the upload tab and upload an image and insert that image.
6. Click the edit button of the checkbox
7. Change the state of the checkbox
8. Save the checkbox
9. Save the content property.

Expected result:

The image is displayed.

Actual result:

No image is displayed.

This is quite likely a regression introduced by the introduction of XWIKI-18810. To be tested on an earlier version.