I just tested on a blank 3.0.5 installation and 3.1 beta 3.
If you have have a DataObject with related SiteTree like this...
private static $has_one = array(
"MySiteTree" => "SiteTree"
);
editing the DataObject... the DropDownField never remembers the selected Page. the same happens with TreeDropDownField.
It looks like it´s always saving the SiteTreeID of the Page that you are just editing in the CMS.
Is there any solution to this? or is it just a bug?
Thanks,
Florian