Undo button does not work after leaving the collaborative editing mode

Steps to reproduce:

• Edit a page in Wisywig mode, collaborative session
• Add some text
• Click on "Undo" button from the Wiki Editor Toolbar
• Disable the realtime editing by clicking on "Leave collaboration" 
• In single user editing mode, add some text

Expected result: to be able to click on Undo button 

Actual result: the undo button does not work after leaving the collaborative editing mode.