Mask not working when using offset effect

Im stumped as to why my text is not masking.

I'm following a tutorial that uses the offset effect so that the text slides up from hiding. But when I reposition my text it still is visible even outside of the bounds of the mask. It seems to match tutorial so I can't figure out why mask isn't working.

This is the tutorial I am following:
https://www.youtube.com/watch?v=g0bK8ug-7PM&t=193s

This is my result: