Not a bug.
The neutral player is a valid player faction which can own tiles, rooms or what ever.
The only thing special about the neutral faction is that all units owned by this faction are instantly converted
to any other faction which comes into view range of these units.
So a claimed neutral tile is a normal thing.
When a tile has no owner it wont be claimed.
In map editor no owner is used by selecting "Terrain" as faction.
The "Neutral" faction is a normal player with some different rules attached.
Could you explain this a bit more in depth?