Consider the following case:

• item 1
  • subitem 1
• item 2
• item 3
  • subitem 2

If both the main list and sublists are connected then root items can be normally sorted each other and item 2 can be dragged to subitem of item 1 or item 3.

However, if you drag item 3 as subitem of item 1, i.e. aiming to get the following:

• item 1
  • subitem 1
  • item 3
    • subitem 2
• item 2

then there will be the following error (and the subitem 2 will not be shown):

Sortable.js:1823 Uncaught TypeError: Cannot read property 'Sortable1551478014931' of null
    at _dispatchEvent (VM1983 Sortable.js:1823)
    at Sortable._onDrop (VM1983 Sortable.js:1446)
    at Sortable.handleEvent (VM1983 Sortable.js:1527)

A bit of debugging showed that ng-sortable triggers destroy which deletes, I believe, the sortable of the dragging item i.e. item 3.

Here is a minimal codepen demo.

This might be related to #43