Steps to reproduce:

1. Create few new Tags under Root
2. Create a Tag underneath one previously created
3. Create Content Type with Taxonomy Entry Assignment with Taxonomy set to Tags
4. Publish Content Item of type with Taxonomy Entry Assignment field
5. Add Tag created in step 2 to newly published Content Item
6. Edit Content Item and add some Tags, but without expanding the tag tree(already added tag should not be visible)

Result:
No new tags are added to selected tags.

Expected results:

New tags should be added.