Hi,
basically, when you have a multi page form, the next and the back button trigger Javascript which shows/hides the respective parts of your form.
Clicking the button does not change the focus at all. This mostly results in the behavior which you describe.
If the parts of the form, which are displayed as "pages" have different length, this does not look so good, too.
At the moment this is, how it works. This can only be fixed with Javascript and in my experience, when you start implementing such a behavior, there are people who would like it otherwise, which means that usually you need some furhter options (don't use the feature, focus on the bottom of the form...), which have to be implemented, too.
Sadly, there is no short and simple advise on how to make this work. It requires additional (Javascript) code.
I will put it on the feature requests list.
Regards,
Aicha