1. Block a person
  2. Join an instance with the blocked person
  3. Sometimes the blocked person sees the blocker but not the other way around.
It could be related that i join an instance of a friend, and that friend is still befriended with the person who i blocked.
But not sure if that is always the case, but it increases the likelyhood of happening since we happen to be in the same instance more often.
There is a very old ticket of this issue here
But it seems it was never looked at.