Lightmapping may need additional setup. Unity’s standard settings were tested successfully on all 3D meshes. 2,5D Meshes may have artifacts. Version 1.01 adds 3 conversion meshes for the low isometric tiles.

Check out the detail from the Unity Asset Store: https://assetstore.unity.com/packages/2d/environments/2d-to-2-5d-3d-conversion-pack-39669

Now download 2D To 2.5D&3D Conversion Pack FREE Unity.