Even though "Clear when enabled" is configured on a trend log, the trend data is not cleared.
The property "Clear when enabled" only affects the trend data when the trend log is goes from disabled to enabled.
This is not the same as the activation method of the trend log. Activation method changes the Status from Stopped to Started etc.
Select the Trend log, right-click on it and select Activation, then select "Disable trend log".
Select the Trend log, right-click on it and select Activation, then select "Enable trend log".