MEDataStore Plus File Header Question

I am running a MEDataStore Plus on a PVP 7. If I have the 'File Headers' on the Advance tab set to "Use tag names", the object works as expected adding rows to the .csv file on each trigger.
If I change it to "Template", it gets the info from the template file correctly but overwrites the whole file on each trigger so there is only one entry row after completion. Is this normal or am I missing something?
Thanks

Not experienced that before.

Alternatively create HMI tags in the root of the system and use these, it will use the HMI Tags exactly as you have typed them instead of the {plc_x::} nonsense, easier to manage if you're struggling with a template.