Seam on a tilemap

It's hard to say without taking a look at the project. Could you please submit a bug report with a minimal reproduction project for this issue and reply in here with the issue ID?

 

This issue was resolved as (By Design):
https://issuetracker.unity3d.com/product/unity/issues/guid/1090209/

Here's the resolution note:
"The seams are caused by the overlapping pixels between neighbouring Tiles. If you turn on Overdraw in the SceneView window (top left dropdown tab in SceneView), you can see that the edges have quite a bit of overdraw (lighter red lines on the edges of the Tiles). If you zoom in with the SceneView in Shaded mode till you can see the pixels in the Sprites, you can try erasing neighbouring Sprites and undoing it to see how the overlapping pixels form the seams.

You can try the following:
-Reducing the overdraw by adding a custom online using the Sprite Editor. Select the texture, open the Sprite Editor and select Custom Outline from the top left tab. In this mode, you can create a better mesh outline by adding vertices from the corners of each Sprite compared to the default mesh generated.
-Adjust the alpha values of the pixels at the edge of the Sprites. Some of the pixels at the edges of the Sprites are transparent. These would blend in the SceneView when overlaid against the neighbours which seem to look like seams."