When you enable Ghost Mode, app sets your «Last Seen» privacy setting to “Nobody”.
After that, application enables «Chats Preview mode». It’s global setting for entire app. Last seen privacy changes only for current selected account. You need to “re-enable” it after you switch to other account.
In preview mode you can enter any chat, but Nicegram will not send read mark to the recipient. In Official Telegram app - You can also preview chats with long-tap (force touch). Read marks will not be send too!
If you send or type ANY message to ANYBODY, your online status may be revealed. That’s how Telegram «Last Seen» privacy settings work!
Ghost Mode feature is just a shortcut for Official Telegram actions.