Steps to reproduce:

1. Import the attached template: a_test_template_v1.xml.
2. Create a new host and use the template on the new host.
3. Modify item and trigger on host.
4. Make a small change to the template (I changed the description-field).
5. Import the modified template (overriding the old one).
6. Check the item and trigger on host. The modifications that were done on the host item and trigger have been overwritten.

Result:
Before:

After:

Expected:
Because Zabbix allows changes to be made on the host level, one would assume those changes would not be overwritten when importing a template. Of course there are import settings such as "Update existing", "Create new" and "Delete missing", but those only control how the importing behaves when applying changes from file into Zabbix templates, not the host items and triggers.