Can't parse a Grid field in RSS feed template

I have setup an RSS feed that fully works with exception of being able to render the author info, which is contained in a grid (flexibility for multiple authors). But I can’t get the RSS template to parse the grid tag pair that works fine in standard template. Each grid row is a relationship field which pulls records from our ‘Author’ channel.

What I have that works fine other than in an RSS template is:

{blog_author backspace="2"}{blog_author:grd_author_name:title}, {/blog_author}

This is EE 2.8.1 btw (yes upgrading is on the list!) 😊