My df has extensible array columns. When I save the df as a csv file, it's writing out (array-total-size column) rows, padded with 0s, instead of (length column) rows of actual data.
Specifically, I create my columns like so: (make-array 0 :adjustable t :fill-pointer t), and then add to them over time. The resulting csv file pads out the data to my implementation's allocated array memory instead of showing only the data in the array.