Skybox rendered over sprites in reflection

My scene has:

A Skybox (under everything):

 "Queue"="Background"
 "RenderType"="Background"

A Unity Terrain:

 "Queue" = "Geometry-100"     
 "RenderType" = "Opaque"

Billboarded Sprites (SpriteRenderer.orderInLayer values are > 0 so they always render above the Terrain):

 "Queue"="Geometry"
 "RenderType"="Transparent"

Particle Effects (over everything):

 "Queue"="Transparent"
 "RenderType"="Transparent"

The problem occurs in water reflections (using Low Poly Water GPU) . The water script uses a separate reflection Camera which renders to a RenderTexture applied to the water surface. Seems straightforward.

Without the skybox, the Sprite plants appear over both Terrain and black sky in the reflection:

With a skybox, the plants disappear. They appear over the Terrain, but BEHIND the skybox clouds in the reflection (I think):

Looking at Frame Debug just confused me further. It lists the draw order as:

 Render.OpaqueGeometry
 Camera.RenderSkybox
 Render.TransparentGeometry

So the Skybox is ALWAYS rendered after Geometry? Then how does it end up behind the sprites aboveground, and over them in the reflection? And consistently behind the Terrain in both cases?

Is this something to do with Stencils? Or in how Skyboxes are connected to Cameras? Or is it due to the Transparent RenderType?