I thought you got a hextech box for reaching mastery level 5 with a champ?

on a different league account this happened might i add.

No, you do not get a hextech chest for reaching mastery level 5, and it has never been like that. You get one when you or someone in your premade group gets an S grade while you are playing a champion that you have not earned a chest with yet.

You are very likely mistaken when it happened on another account, as you most likely got an S grade at the same time you reached mastery level 5.