Plone-mosaic.css not referenced in anon view?

I just noticed today that my site is only presenting a single column view to anonymous users.
In testing, I noticed that when I'm logged in, the stylesheets include plone-mosaic.css however, this file is not referenced in an anonymous view.
I can still access it if I include the direct URL (in this case is so it's not a permissions thing.
However, I can't figure out when this started happening, and if it's related to my upgrade of Plone to 5.1rc and Mosaic 2.x?
The old ZMI system of debugging the css doesn't seem to do anything any more, and I'm not quite sure where to look for an answer to this!
Thanks for pointers!