Steps to reproduce:

1. When I change a already defined preprocessing step (Discard unchanged with heartbeat) from 1h to 600s in a Discovery Rule -> Item prototype (for example in the "MySQL by Zabbix Agent 2 Active" template for the item "Size of database {#DATABASE}".
2. The change is saved and updated in the original template.
   
   
3. Linked template also gets this change.
4. Already discovered items (databasenames) don't get the new updated interval for Discard unchanged.
5. After waiting a few hours, no new settings are propagated to the already discovered items.

How did I get this update of the preprocessing steps propagated to the already discovered item?

1. Change the 1h to 600s in preprocessing step.
2. Also change something else in the item (for example add some extra text to the Description of the item).
3. Only then, the new values were propagated to the already discovered items on my database servers!