I hadn’t seen this mentioned anywhere else, but maybe I missed it. Just picked up the Headband of Intellect. It now changes the wearers Intelligence to 17 instead of 18. That is a good change in my opinion. It’s great for Lae’zel to use as an Eldritch Knight, but would be too weak for a Wizard to use. More appropriate.
It is supposed to be 19... like other stat altering magic items. The idea being that it isn't 20, which is what you would really want it to be as a primary attribute for a class, like wizard in this case.