I just noticed this may have nothing to do with the Wordpress mystery person avatar and might just be what Discourse shows if you choose “Custom picture” without actually uploading a picture. Coincidentally, they look the same.
I’m not sure why anyone would do that but if that’s the case then it’s probably not related to the Wordpress plugin. 
It’s strange though that some users’ preference would get changed from “System assigned profile picture” to “Custom picture” without them changing the preference.