It is kinda the bug I hoped I fixed long ago.
Assuming you are on the latest version of EVE-O Preview - could you create a screenshot of this issue and send it to me via EVE mail? It will be kept private.
As for the option - it is expected that previews to fall behind other windows when the ‘stay on top’ option is not checked. However failing behind when the option IS checked is kinda another bug I have to look at.
Again please confirm that you are on the latest version of EVE-O Preview (it is really important here)
TIA and sorry for the inconveniences caused