How to reproduce:

1. on a dashboard create tabbed panel using 3 panels as tab data sources
2. open tabbed panel for editing
3. change panel for 1st tab
4. save changes

What happens:

The tab where panel was changed is completely removed.

What should happen:

The tab should stay.

Phabricator Version: https://secure.phabricator.com/w/changelog/2017.20/