Data flow sink to Blob storage not writing to subfolder

Hi Everybody

This seems like it should be straightforward, but it just doesn't seem to work... I have a file containing JSON data, one document per line, with many different types of data. Each type is identified by a field named "OBJ", which tells me what kind of data it contains. I want to split this file into separate files in Blob storage for each object type prior to doing some downstream processing. So, I have a very simple data flow - a source which loads the whole file, and a sink which writes the data back to separate files. In the sink settings, I've set the "File name option" setting to "Name file as column data" and selected my OBJ column for the "Column Data", and this basically works - it writes out a separate file for each OBJ value, containing the right data. So far, so good.

However, what doesn't seem to work is the very simplest thing - I want to write the output files to a folder in my Blob storage container, but the sink seems to completely ignore the "Folder path" setting and just writes them into the root of the container. I can write my output files to a different container, but not to a subfolder inside the same container. It even creates the folder if it's not there already, but doesn't use it.

Am I missing something obvious, or does the "Folder path" setting just not work when naming files from column data? Is there a way around this?