I have noticed that some users are getting a bluish Gravatar which looks like the following:
Is this intentional or just a bug?
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When I visit those user profiles on the main site, their profile image returns a 404.
The ones I checked were all hosted by Google on the lh[3|5|6].googleusercontent.com domain. It could be that there is a temporary outage or that the user decided to remove that image / deleted their Google account or Google stopped offering the service.
This is a 3rd-party problem and not something Stack Exchange, or you for that matter, can fix.
If this is permanent the impacted users need to take action, for example by uploading a new profile image, preferable into the imgur instance of Stack Exchange.