Do you have sliding fields in this layout that resize the enclosing part?
Yes I do and they are sliding and appear to be working correctly other then not filling the page.
Then regrettably, it would seem that you have encountered a known bug.
For More Information see: Frustrating subsummary bug
This is one of many acknowledged bugs that can be found in the Known Bug List thread here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: https://www.dropbox.com/s/jt09b82i0xijbu3/FMP%20Bugs.zip
That's unfortunate. I'm surprised this hasn't been addressed. It's been known for some time now. Ok well it is what it is. I noticed in the thread that if you make your parts smaller it gets better. I did that and yes it got better but certainly far from perfect.