I had this problem too. But, I managed to solve it.
I had deleted empty folders in my start menu. Apparently, not having a Startup folder in the Start Menu causes the new Task Manager to not show any startup items. Re-creating the empty Startup folder in my Start Menu made the other startup items reappear
in the list for me.
So, make sure you have a Startup folder under Programs in both your personal start menu (default of "C:\Users\_username_\AppData\Roaming\Microsoft\Windows\Start Menu\Programs\Startup") and the all users one (default of "C:\ProgramData\Microsoft\Windows\Start