Infinite scroll does not respect “tag” query var

I set up a blog post filter in my theme so that you can filter posts by category or tag. I update the infiniteScroll.settings.query_args values via Javascript when the form is filled out. This works fine for infiniteScroll.settings.query_args.category_name but not for infiniteScroll.settings.query_args.tag (or any other permutation of the tag query vars, e.g. tag_id, tag__in, etc).

I’ve confirmed that the XHR request is sending the correct query vars and is being received by Jetpack, but it seems to ignore the tag entirely.