I read that if the material of label is not NULL, don't set material again.
Back to the example, first I set text = "abc", I think the material is only contain images of "a", "b" and "c"
Then, I set text again = "adef", the material not contains "d", "e" and "f", so it can be showed "a" only.
Other example, if we set text = "adeb" at step 2, it shows "a b"