Steps to reproduce:

Assuming one has a host were the template is assigned. The template is an item with a tag (item1).
Then it is decided to add a host level tag for that item (hostitem1) So now this item has tags (item1, hostitem1).
Finally, somebody else adjusts the templated item tag to (modifieditem1).

Result:

As a result Zabbix silently REPLACES all tags and the host item now has only (modifieditem1)
It breaks every Actions/Services/Reports and anything else that relies on tags.

The same is true for triggers, item and trigger prototypes, and httptests.

Expected:
Zabbix keeps all unchanged and adds new tags.