Content from Realtime Editing session aren't stored in the page content after it has finished

Prerequisites: have clean version of 15.8, be the admin of your XWiki. Create a normal user. To better visibility upload different avatar images for both users. Install latest version of Realtime WYSIWYG Editor. Set Realtime CKEditor as Default WYSIWYG Editor (XWiki Preferences → Editing → WYSIWYG Editor). Last have more than one web-browser client.

Step to reproduce

1. Open a new tab in private-mode (or normal it's also reproduced)
2. Log in as Admin user, enable advanced user feature
3. Go to any desired page (Sandbox)
4. Press "e" for editing and start editing (see message in CKEditor menu toolbar "Editing alone" with your custom avatar)
5. Open a new tab in private-mode but in another web-browser
6. Log in as secondly created user (normal), also enable advanced user feature
7. Go to the same page as admin
8. Press "e" and click "Force editing" in warning message
9. Start editing with normal user (you'll see both avatars and different "Editng as Admin" and "Editing as user")
10. Perform saving (click "Save" button) by admin user after typing a simply line sentence, then do the same with normal users (don't forget to add summary before saving)
11. Leave a collaboration session by one of the user, press "Save & View" by left user

Actual result
Both users will see the original page state as it was before collaborative editing. However, the saving records will be present in History tab with content from this editing.

Expected result
The page content is updated with content from collaboration session.