Steps to reproduce:

Scenario #1 Existing tag is replaced by the new one

1. Open edit form for any existing item with tags (both item and inherited) on any template
2. Open tags tab
3. Switch to "Inherited and item tags" tab
4. Add new valid tag
5. Update the item
6. Re-open the item and see the tags

Result
The previous tag is replaced by the new one

See video:

Expected
New tag should be added, previously existing tag should be present.

Note: issue is also reproducible for templated items on host