If the text box is not "disappearing" but is always visible, have you tried setting the initial Stagger Animation keyframe to "-100" (negative 100) and the ending keyframe to "0"?
Stagger Animation's alpha controls are set to work in conjunction with the existing alpha parameters of the text boxes. For example, if the alpha of your text box in the Object Manager is set to an alpha of 100, you need to set the Stagger Animation's alpha to -100. That way they counter-act each other and your text's alpha property should appear as invisible or "0".
Likewise, if you set your Keyframe to "-50" and your Text Box's Object Inspector Alpha is at 100, it should display at half opacity. Setting your Keyframe to "0" will not affect the current alpha value in your Object Inspector at all.
Hope this helps!
#XPression