There are some differences in using a portals in Go. Issue have been reported as a bug for similar behavior, for some reason a portal with scroll bars wants to stay in front. I don't know if it is official consider a bug by filemaker. PhilModJunk talks about the behavior of a portal with scroll bars (dealing with sliding objects and hiding objects) in his post at this link, which also has a databases of the know bug list.
Thanks, I can confirm that this kind of bug only exists for a portal WITH scroll bars. The button stays at top when removing scroll bars on FM Go.
This is an example of a known bug.
For More Information see: FileMaker Go
This is one of many acknowledged bugs that can be found in the Known Buglist for Filemaker GO 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
The bug report thread refers to a tab control, but any layout object you might try to put on top of the portal will not be covered by it.