Steps to reproduce:

1. Create template derived (e.g. "Template Net Cisco IOS SNMPv2 - CUSTOMIZED")
   from existing template (e.g. "Template Net Cisco IOS SNMPv2"), by template linking.
2. Optional step: You may now customize new template.
3. Assign customized template to host (hosts).
4. Try to customize some item (e.g. zabbix[host,snmp,available]) in customized template, e.g. update interval.

Result:

Expected:
Item update in intermediary (customized) template. This works when no host assigned to customized template.