In video 5_GG_CBC while selecting multiple stairs, I was trying to rotate a duplicated block. This does not appear to work as expected, only the last selected item rotates at all. I found a few online items that would make this better ( parenting to an empty object, changing pivot point, etc ), but these are well past basic rotation as discussed in the lecture.
I hope you found a solution to your problem, but if not, here is my best guess at what is going on.
Based on your issue and what I see, it looks like you are changing the rotation in the object data, which only works on the primary selection.
It should work if you press R or use the rotation gizmo.
As I recall it the last person with this issue was trying to rotate using the numeric inputs in the N panel, which can only affect one object at a time, the active object. For all to rotate together you use the usual shortcuts and type an angle in if not just dragging the mouse. So R Z 35 or some such.