Actually, it does work if you do it in the following way.
On the dashboard widget, click the button to go back 1 week. Then open the wieght page and you will see that the arrows do work if you do it that way. The only time it doesn't work is if you are in the current week when you start. It's still somewhat of a bug that the back arrow doesn't work like it should but, at least, there's a workaround.
Actually, it does work if you do it in the following way.
On the dashboard widget, click the button to go back 1 week. Then open the wieght page and you will see that the arrows do work if you do it that way. The only time it doesn't work is if you are in the current week when you start. It's still somewhat of a bug that the back arrow doesn't work like it should but, at least, there's a workaround.