Steps to reproduce the issue

Create a new tag and try to select a "parent"

Expected result

The "parent" list to show the existing tags.

Actual result

The "parent" list is empty

System information (as much as possible)

4.0.0-beta4-dev

Addition

Possibly this is a general issue with the comboboxes. I am having the same issue, when i try to select tags inside articles and in the comboboxes inside various modules.
The options show only after typing (searching) them.