Why python driver does not change texture offset?

I would like to know if it’s possible to change the texture offset of a material using a driver.

By having a look on this tutorial it seems very easy.

I have put a very simple example that simply will scroll the offset based on the running frame. I don’t understand why it does not work. The offset looks like it is updated only on user input.

test face change.zip (92.1 KB)