Content added in Source editing mode can get lost when editing in realtime if no other modification is made after the realtime session is joined

Steps to reproduce

1. Login as Admin and enable Realtime editing from Administration
2. Edit a page in-place
3. Click on 'Source' button
4. Insert some text (e.g. 'Admin's Source')
5. Click again on 'Source' button to re-join the realtime session but don't make any other action (like inserting a space or another character)
6. Open the same page in an Incognito window or another browser
7. Login with an user (e.g. U1)
8. Edit the page and insert some text with the user U1
9. Wait for the automatic save to occur

Expected results

On automatic save, the user U1 gets the Conflict Warning modal, since before his editing, there was Admin's content which was not yet saved.

Actual results

Only the content added later by the user is displayed, the content added initially by Admin in Source mode is lost.

I think the issue is that when the user U1 starts editing the page, the content added by Admin in Source mode is not displayed on the page, which does not happen if a modification is added in WYSIWWYG after the realtime mode is joined.