I have this fixed for the update. Thanks for the report.
I simply just have to force a
/show_window MainHUD.StartMenu on that button now. The only quirkiness is that the menu shows next to where ever your XP bar is located (regardless of it having the integrated button or not).
I could always swap the start button code into the experience bar, and make the old start menu window the experience bar
But maybe they'll fix the old button since it's still included in the UI...