This is a known limitation to using sliding fields in FileMaker. It's been that way through many versions of the application.
One possible work around is to base your layout on the portal's table and use a list view layout with the needed records from the parent table included in the header and other parts of yoru layotu as needed. This is how the FileMaker invoice starter solution prints an invoice if you want to see an example.
With the fields thus removed from the portal, slide up/resize enclosing part will now work.
Key facts about sliding layout objects:
- It's only visible in preview mode and when you print/save as PDF...
- Sliding fields will shrink but not expand.
- All layout objects below and in the same layout part as the slide/resize field need to also be set to slide up and resize.
- Objects in headers and footers will not slide.
- Portals will shrink/slide to fit the number of rows of records, but fields within the portal row will not shrink/slide.
- Fields will slide up only if Top alignment is specified for it and will slide left only if Left alignment is specified.
- Consistent side borders are difficult to achieve with sliding fields.