Forgot to add that I'm on FMA 12.0v4
I have this problem on Windows 8 as well, but I have had the issue with both image and text fields. Also experienced the bug if I copy the portal to another layout. After paste, I have to select the row, reduce the object height to one point smaller, then one point larger again.
Actually its probably intended behavior. (I'm sure someone desires it).
In FMP12 you can now place an object above a tab and have it floating above/outside of the tab layer. Moving it one pixel is the method used to move the object into the tab layer.
If your Portal is in a tab you could be expieriencing this behavior.
If your portal is no in a tab then possibly the original alignement of the field was not fully within the portal.
My portal is in a tab, so this is probably the case.
The tab is not specifically the issue...
This likely has to do with the Parent Child relationship established between objects. In FM11 and prior portals did not encapsulate the objects within them, Like Tabs do. In FM12 Portals now link the objects as children and the objects move with the portal. It probably has to do with the object layering,... when you physcally drop the object on the Parent (portal or tab) it knows to link it as a child. Changing the position value of an object outside the parent/child link, in the inspector does "not" establish the Parent/Child link.