Comment on WordPress Gravatars by SEO Dave.
Not sure which WordPress update caused the problem, I think WordPress 4.2, but could have been 4.1.
When the Stallion Gravatar cache is on (set under Stallion Theme > Performance Options : Gravatar Cache ON) Gravatar images that are associated with an email address (a real image like the Stallion horse image I use) are cached correctly. However the computer generated images (these are randomly created for email addresses that don’t have a gravatar.com account) are not caching correctly, rather than making a cached image of the generated Gravatar the default blue Gravatar Logo is saved instead.
So far not been able to figure out what’s gone wrong, when I do will put an update out. I don’t consider it a major bug, it’s OK to keep the gravatar cache feature active (it’s still on this site, you’ll see new comments without real gravatar images use the blog logo).
Took me a while to realise there was a problem, seeing a few blue Gravatar logos didn’t look out of place. I’ve not made any changes to the feature since adding it, so reasonably sure something must have changed with either WordPress or Gravatar.com to cause the issue.