When you clean history from frontend for item with discard unchanged preprocessing, the history can stay clean virtually forever (until next change in item value, disabling/enabling item or next Zabbix server restart)

I understand that there are performance reasons for such behavior, but sometimes this is very annoying when testing such items (need to restart server or disable item, reload configuration cache, enable item, reload configuration cache)

It is not convenient for inexperienced person how to handle such issue quickly 

Maybe Execute now button can check the historical table and populate the value if the table is empty for this item ? This must not have serious performance effect, because such checks are executed manually and mostly for testing purposes.