Steps to reproduce:

• Create a trapper item, send a value to it, make sure values stored to history DB table.
• During the same hour do clear item history/trends from the frontend.
• Verify in the DB that the corresponding trends table has no values for this item.
• On next hour later send another value - a trends record from previous hour will appear. OR terminate zabbix_server now and the trend value will be inserted right away.

So, basically the option in frontend does not clear trends for sure at the point of time when it's performed.