Interesting. I see the same thing for the mule if I hover as I posted. (no last seen) But if I hover over most other users I do see it. So I assume that the mule has set his privacy options so that we can't see that. (But you as a mod still can.)
Looks like I've learned something new, never too late for that. Apologies to @TheTalkingMule for (inadvertently) bypassing his privacy settings and revealing his last seen date.