but the setting to only download PAR files as needed was still not working.
If they don't group properly, there's no way to know if files are still downloading for a set. Newsbin will download "in group" pars with each retry failure in hopes of eventually completing the files. When a set of files and pars group, Newsbin knows when the last file has downloaded so it can then download pars as needed. That's why force grouping these fucked up NZB files solves this issue.
The primary goal is to finish downloads. Saving bandwidth by not downloading pars is a secondary goal. If something unexpected happens, like you're trying to download ungrouped sets, you may end up with too many pars.