I haven’t used FL Studio, but what I think:
Drag-and-drop can be very different things, a text-snippet, a graphic, a file or anything else program specific. It is defined by a mime-type, that is generated when a drag starts.
When it works in FL-Studio, but not to drag it out of FL Studio, it probably means, that they don’t drag a file, but rather their own specific information.
You can easily test it, if you drag from FL Studio into a finder/explorer window. If they start the drag as file, the finder will offer the usual copy/move options. If not, you know, that the FL Studio file browser is not meant to be used for exporting files.
And to double check that test, do the same from Studio One.