here is preprocessing/cache bug:  If you have items with "Discard unchanged" preprocessing, and you restart zabbix-server, item history can be updated with recieved value against "Discard unchanged" settings.

Expected:
warm cache with preprocessing values on server start or select values from database and avoid updating "Discard unchanged" items without real need (it broke item last update time).

https://zabbix.org/wiki/Docs/specs/ZBXNEXT-322 If historical values are not present in the cache, the missing values will be requested from the database and cache updated accordingly.

Example:

I have "Module Interfaces SNMPv2" template attached to switch. It has "Operational Status" items with "Discard unchanged with heartbeat"=3h. And I want to know date when interface on switch change state last time. I can looking for history/graphs, but for 24/48 ports it very annoing. Thus I create ifOperStatus dependend item "Last change" with "Discard uncahanged" preprocessing. All work ok, depended items updates only when switch interface change status and I can see it as "Last check" date in Lastest Data.

Then once I restart zabbix-server, it suddenly update all of depended items with current date and recieved (99% the same) value against "Discard unchanged" preprocessing. So all my Dependen items became useless instantly.