I have a PR created to address this issue:
https://github.com/discourse/discourse/pull/10350
While looking into this I discovered that the .rss routes don’t actually call the same backend code as the .json routes do so this is why the behavior is a bit different and was ignoring the query parameters. I’m not actually sure they the .rss routes are different, but my change makes sure that the /latest.rss route will take into account any query parameters. If there are any other .rss routes you are hitting we may need to update those as well.