Simulation-based color layers are dependent on the layout of the tile they're affecting. In your case, the Flow effects are flowing from top-to-bottom of the tiles, and when you subdivide them that creates flow artifacts at the tile seams. You can either place the Terrain Color operator above the Terrain Tile operator, or you can use a more advanced tile blending workflow.
Also worth noting: the artifacts are very visible in your screenshot because your terrain is perfectly smooth. Once you perturb the terrain with noise/erosion/etc, they will become much less visible.
In the scene attached you can enable/disable the Terrain Transform operator at the end to see the individual tiles - notice how their seams are virtually invisible (due to the use of the advanced overlap blending mode of the Terrain Tile operator, before/after the addition of noise and color).
Also worth noting: the artifacts are very visible in your screenshot because your terrain is perfectly smooth. Once you perturb the terrain with noise/erosion/etc, they will become much less visible.
In the scene attached you can enable/disable the Terrain Transform operator at the end to see the individual tiles - notice how their seams are virtually invisible (due to the use of the advanced overlap blending mode of the Terrain Tile operator, before/after the addition of noise and color).