Ok, was able to confirm your issue with the below scenario.
User A Blocks User B
User B views their Last Visitors Tab
User A on User B's Last Visitors Tab is blank
That's the issue described in my reply above.
www.joomlapolis.com/forum/255-developer-members-support/242925-user-is-blocked-by-moderator-css-is-shown#320565
I'm currently working on a fix for this, but typically that's solved by using a display override for formatname, avatar, and canvas fields. I however am implementing default display overrides so issues like you're seeing can't happen anywhere.