I thought I was having a stroke this morning when I couldnt access the checked property of a menuitem I had just added. It turns out I wasnt having a stroke but that the 2017r3 IDE is automatically adding an index to a menuitem whenever you change the name. It really should only do that if you change it to a name that is already in use I think. The 2017 r2.1 IDE works properly and does not do this.
The work around is easy as you can just delete the index that it inserted, but it will cause you some soul searching as you try to compare what youre seeing in the 2.1 IDE that works with what youre seeing in the 3.0 IDE.
This basically breaks every example using menus in the documentation but keeps working fine with any menus youve already created.