Is the portal itself transparent or have you selected a fill color for it?
When I test this on a portal on my XP System with FMP 11 advanced, the transparent field shows the fill color of the portal, if one is specified. It only shows the layout background color if the portal's fill pattern is transparent.
No, the portal is not transparent. I went ahead and updated the original screen grab to also show an example of a transparent fill field on top of a filled rectangle. The middle picture shows what I would expect to see with a transparent fill field (the rectangle's color around the field) but if I activate the field, it takes on the background color instead of maintaining the color of the first non-transparent object beneath the transparent field.
In - Layout Menu - Layout Set Up - uncheck - "Show field frames when record is active"