Mask: Apply only does something when you add another filter in between the 2 mask filters:
If you remove Mask: Apply then your “Mask” is defining a region of transparency like a window, but you have not defined what is on the side of the window you are looking through (something on a lower video track).
There are simpler ways than using Glaxnimate to draw a rectangle. It depends on if you are going for a typewrite effect or not because a typewrite effect across multiple lines is difficult. If you simply want to wipe the text like a transition you could add a color clip before it and overlap the color and text clips to make a transition and change the wipe in transition properties to Bar Horizontal.