Hello,
The On site editing was designed to have the real time preview without need going to CMS Desk and have the ability to edit the site. So, you need to disable the
on-site editing feature.
As for the preview mode - I would recommend using different browser or clearing the browser cookies. In this case the CMS Desk was opened in the browser some time before or is open in a different tab - since the browsers are sharing the cookies, the view mode is set to "preview" or "edit" mode.
Best regards,
Juraj Ondrus