Please show us what you're dragging and where you're dragging it to.
The FLATVIEWCOPY argument only affects dragging out of FlatView. Sometimes people expect it to work with File Collections as well, but it doesn't affect those.
You can also make the same change via Preferences / File Operations / Copy Options, without editing the drag & drop command at all. In fact, it asks by default, so you should not need to do anything extra unless you've told it not to ask in the past.
If everything being dragged comes from the same folder/level, it may treat that as a normal drag. (Haven’t looked at the code or docs to check, but it would make sense.) The argument/option is about what to do when dragging things that aren’t all in the same place.