When breaking your mysql historical partitions, you cannot add a given sets of data to your mysql database.

[Z3005] query failed: [1526] Table has no partition for value 1425297600 [insert into trends_uint (itemid,clock,num,value_min,value_avg,value_max) values (1907943,1425297600,1,124427,124427,124427),
...

Ok, that's natural to break your data if you cannot add to your database. And i already monitor zabbix_server.log for errors like this.

But your data gets flushed out and cache is not used to store it.
Zabbix Server knows that it did not insert successfully to the database, so it should hold that data in History Cache until it can safely store it. This behavior would then fit perfectly with the description of History Cache (text and uint) usage:

"History cache stores item and timestamp information for all item types as well as value for the numeric types. A low number indicates performance problems on the database side."
From https://www.zabbix.com/documentation/2.4/manual/config/items/itemtypes/internal