Save warning about merge when creating new page and adding an image

Steps to reproduce:

• create a new page
• edit the content in the standard wywisyg editor, before saving the page (in the editor displayed after hitting "Create" in the create form)
• upload an image, using the image menu of the wysiwyg editor or by pasting the image from the clipboard (in supported browsers)
• Save

Expected result:

• the page is saved with the typed content and the image

Actual result:

• a version conflict dialog is displayed upon save
  

Doesn't reproduce on a 12.6.2 so it's a regression.