Hi Ben,
From following the link you posted, it looks like you already found a means to resolve this. However I would agree with you, I don't see how the steps you took would have effected the avatars displayed in your browser session.
If this is an Atlassian Cloud hosted instance of Jira, then I would actually recommend creating a support request on https://support.atlassian.com/contact/ should you still have users on your site that cannot see this.
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