Drag into "+Add Block" is inconsistent

Builder content link

Builder public api key

Detailed steps to reproduce the bug
How to Add Dynamic Editing Regions to Custom Components is helpful here

  1. create a Symbol with multiple Slots. For example, a Columns object with 2 columns and one Slot in each. Give the Slots different names (e.g. mainChildren and leftChildren) so they don’t share the same content.
  2. create a Page
  3. add an instance of your Symbol to the page; don’t add any children
  4. add some other content oustide the Symbol
  5. try to drag the other content into either of the “+Add Block” targets. Sometimes the content adds as a child; sometimes it moves to before or after the Symbol.

This happens in both the embedded and fallback editors.

Screenshots or video link
This one failed:
As did this one:
But this did what I wanted: