I can reproduce. The problem is that LyX does not know the refresh the menus for some reason. So when you start a new document, the menu options available are as if you had no document open. I'm not sure why this is happening.
I have found a (partial) workaround: if you press "alt" in Ubuntu it brings up the HUD which searches the menu of the open application. For example, if I press alt and then "sett" it shows me "Settings (Document)", meaning that it found an entry under Document that matches my search string. I can then press return and it is as if I went to Document > Settings. In LyX, after I open the HUD and type a letter, all of the menus are refreshed. However, you have to do this each time you want the menus to be refreshed.
I can reproduce. The problem is that LyX does not know the refresh the menus for some reason. So when you start a new document, the menu options available are as if you had no document open. I'm not sure why this is happening.
I have found a (partial) workaround: if you press "alt" in Ubuntu it brings up the HUD which searches the menu of the open application. For example, if I press alt and then "sett" it shows me "Settings (Document)", meaning that it found an entry under Document that matches my search string. I can then press return and it is as if I went to Document > Settings. In LyX, after I open the HUD and type a letter, all of the menus are refreshed. However, you have to do this each time you want the menus to be refreshed.