This LosIsOffWorld check prevents explored tiles outside of the world from being counted in the cache m_ExploredVertices for rejoined clients.

The absence of a LosIsOffWorld check here means that the tiles outside of the world that are (incorrectly) marked as explored here are added to the m_ExploredVertices cache.

Hence rejoined clients have a smaller number of explored tiles than non-rejoined clients returned by GetPercentMapExplored if the initial territory of a player extends outside of the world (what it shouldn't do either), see also #4598.