Once an element was added as a child of another one, its alpha will always follow its parent's one. And also, this only occurs when I re-change the parent's alpha.
Like there are two labels whose names are A and B. I made label A a child of label B. Now if I reset the alpha of label B to a different value, the alpha of label A will also be changed automatically to the same value as that of label B.
What you were doing when the bug happened
I was changing the alpha of parent's element.
What you think caused the bug
One element being a child of another one