Minecraft maps

Hello I'm a player in the Minecraft world and I'm here to report a bug in the Minecraft Java Edition 1.16 snapshots update. This bug is the Minecraft map item. Whenever I create a map using the empty map item, there are usual lines across the map, these are not placed blocks or items in the world. Sometimes the lines are different colours. I don't know if this is showing the underground too or it's just a bug. Please fix this.

Thank you.