Pages dissapear when rearranging Parent > Child to Child > Parent

[peda]

Describe the bug
When reordering pages and change Parent > Child to Child > Parent the pages disappear from the admin panel

To Reproduce

1. create page parent
2. create page child under parent
3. now reorder pages that child is the new parent and parent is the new child

see: https://www.loom.com/share/282015313fe54be6b53f6c7b5414779c

Expected behavior
Pages should be reordered or an error should be shown if operation is too complex - but they should not disappear

Screenshots
https://www.loom.com/share/282015313fe54be6b53f6c7b5414779c

Environment details (please complete the following information):

• Statamic Version 2.11.20
• OS: Ubntu 18.04
• Browser: Chrome
• Web Server: Apache
• PHP Version: 7.1