Steps to reproduce the issue

Create some tags (e.g.: Tag1, Tag2, Tag3)
Edit an item (Article, Contact... whatever is "taggable")
Add one or more (not all) tags (you can select between Tag1, Tag2, Tag3)
Save the item
Edit that item again
Try to add a new tag

Expected result

You should see a list of the "remaining" (not assigned) tags

Actual result

You see a list of the tags already assigned to that item

Workaround

Use batch tagging