Start by dropping on Content a SplitPane.
When you hover the Hierarchy with a second SplitPane and come over the bottom AnchorPane a ring encloses the AnchorPane: this is DropPosition1.
Now if you move down one or two millimeters you get DropPosition2.
And if you move down again by another millimeter or so you get DropPosition3.
Fine: the problem is that once in drop position 3 if you move up you'll never get drop position 2 but always drop position 1.
Drop position 2 is reachable only from top to bottom, never from bottom to top.

You can reproduce the issue with any other container: in top AnchorPane drop a second AnchorPane then a third one within the second one. Now when hovering third AnchorPane with any item you experiment it all lthe same.