Sort order not updated

[JamesDPC]

It's possible for the sort order to not be update-able after an element is moved.
There is some funkiness in the way the base elemental UI tells the backend what sort value to use.

In this example Element 2 (with a sort value of 1) was moved into the area containing element 1 and 3:

+----+----------+-----------+------+
| ID | ParentID | Title     | Sort |
+----+----------+-----------+------+
|  9 |       62 | Element 1 |    1 |
| 15 |       62 | Element 2 |    1 |
| 16 |       62 | Element 3 |    2 |
+----+----------+-----------+------+

It's only possible to re-order Element 3 in the UI and have that retained, with these sort values resulting:

+----+----------+-----------+------+
| ID | ParentID | Title     | Sort |
+----+----------+-----------+------+
| 16 |       62 | Element 3 |    1 |
|  9 |       62 | Element 1 |    2 |
| 15 |       62 | Element 2 |    2 |
+----+----------+-----------+------+

[JamesDPC]

Possibly similar to silverstripe/silverstripe-elemental#1046