In version 3.8.1 it currently appears that comments from WordPress and Micro.blog (both of which have Gravatar as a commonality) appear to be automatically using the default mystery person avatar even though there is a separate avatar defined within the Avatar fields for Semantic Linkbacks Data.
If I add my my email address within the comment editing interface, then the appropriate Gravatar is pulled and displayed as expected. This leads me to believe that somehow SL either isn’t finding/pulling the Gravatar URL that it’s storing, or the code is potentially skipping looking for it within its logic structure.
I’ve noticed this also seems to be the case for other WP sites which I have reason to suspect are running this same version as well.
Webmentions from other non-Gravatar related sites don’t seem to be affected by this and they’re displaying the proper avatar images as defined within their h-cards.