On 1.5.2, you can only use the CSV merge import mode when you are in the table window. When using the extension via the inspector, it completely clears the table.
We are using custom CSV columns so we need to use the inspector extension. It would be nice if the merge import mode would be available there too. I’ve found a workaround by calling the CSV.ImportInto manually with removeMissingEntries to false. But I think this should be a default option.