Hi,
Yes, it is related to the subforms. What is happening is that there is content than extends beyond the bounds of the page. This is not helped by setting up the master page content to the full extent of the page size. You would normally allow a boundary and then work within that.
Attached is the form, but you should check it out to make sure it is working as you intend. By re-organising the subforms I have got rid of the extra pages.
I would recommend that you call master pages with a capital "P" and body pages with a small "p" as it helps to distinguish them. Then name the subforms differently.
Where you want a group of object to expand in height, you should wrap these in a subform and set that to flowed. Page 2 is an example. The objects are not grouped in a subform and I am surprised that they are maintaining their position even though page2 is set to flowed.
Where objects do not increase in height, these can be wrapped in a separate positioned subform. Provided the page is set to flowed, the positioned subform will still move downwards as objects above it increased in height.
So, I would recommend tidying the structure a small bit.
Good luck,
Niall