Haha - now you are asking the hard questions!
OK, so when you use the “+” you are doing a concat and that operation returns a new list. When you use append, it mutates the list in place but doesn’t return anything. Or more precisely, it returns None
So when using the append in the callback, it was returning None (even though it was updating defaultCondStyle) . Returning None to the style_data_conditional property will remove all of the formatting. That is why the background colors went back to the default white.