SpriteKit texture atlas leads to image artifacts and blank areas - swift

Question

I have a SpriteKit game in which there's a shape created via SKShapeNode(splinePoints:count:).
The shape has a fillTexture, loaded from a texture atlas via textureNamed(_:).
In my texture atlas there are 5 images -- 1 of which is used for the shape.
If I use the regular .xcassets folder instead of an atlas, the shape is textured correctly. So it's definitely an issue with the atlas. Also, the texture works correctly if it's the only image in the atlas. It's when I add additional images that the problem occurs.
Here's the code that results in the correct texture:
var splinePoints = mainData.getSplinePointsForGround()
let ground = SKShapeNode(splinePoints: &splinePoints, count: splinePoints.count)
let texture = SKTexture(imageNamed: "groundTexture")
ground.fillTexture = texture
ground.lineWidth = 1
ground.fillColor = UIColor(Color.purple)
ground.strokeColor = UIColor(Color.clear)
addChild(ground)
Expected results:
The shape, which has a purple gradient image, should look like this (please ignore the dotted white line):
Actual results:
Instead, the shape looks like this, with strange blank areas and little artifacts from the other images located in the atlas:
Here's the version of code that uses the atlas:
let textureAtlas = SKTextureAtlas(named: "assets")
var splinePoints = mainData.getSplinePointsForGround()
let ground = SKShapeNode(splinePoints: &splinePoints, count: splinePoints.count)
let texture = textureAtlas.textureNamed("groundTexture2.png")
ground.fillTexture = texture
ground.lineWidth = 1
ground.fillColor = UIColor(Color.purple)
ground.strokeColor = UIColor(Color.clear)
addChild(ground)
Why is this problem happening, and how do I resolve it?
Thank you!

Answer 1

It looks like I've solved the problem, though I'm not sure why my solution works.
Instead of having a top-level .atlas folder, I've put a .spriteatlas folder inside Assets.xcassets.
For whatever reason, this results in the correct textures being shown without any nasty artifacts or transparent areas.