Describe the bug
Context: on a website with a lot of pages on 0/1/2/3 or 4 sublevels depending on the 1st level page, using menu order on first level but also sometimes on sublevel, the dropdown duplicate some pages and doesn't show all.

Debugging:

1. i duplicated the selector and changed the query to sort by ID, it's working fine. So to make it work change orderby menu_order to orderby id
2. the bug is in the function "buildTermsTree" here

   gutenberg/packages/editor/src/utils/terms.js

   Line 13 in ee6060f

   export function buildTermsTree( flatTerms ) {

3. after logging a bit it seems to be in the use of groupBy (from lodash) here:

   gutenberg/packages/editor/src/utils/terms.js

   Line 14 in ee6060f

   const termsByParent = groupBy( flatTerms, 'parent' );

In my tests, the param flatTerms looks fine in both cases, but termsByParent is different

To Reproduce
Steps to reproduce the behavior:

1. Create 300 pages on different sub levels
2. Set menu order on some of them
3. Edit a page and try to choose en parent page
4. See what is the result

Expected behavior
No duplicate content

Screenshots

Additional context

• This appears in the last version of Gutenberg (4.7.0 RC and 4.6.1 on Wordpress 4.9.8 and on Wordpress 5.0)