How to fix 'procedural skybox' visible horizon line on 'screen space camera' canvas?

I can see the horizon line through my screen space camera canvas.

Here are the steps to reproduce it for 2019.3.0f3:

1. create a new scene

2. create a new material for the Skybox

   • set shader to Skybox/Procedural
   • set Atmosphere Thickness to 4

3. open Window/Rendering/Lighting Settings

   • set Skybox Material to material created in step 2
   • click Auto Generate at bottom

4. in Hierarchy add a new UI/Panel

   • set canvas Render Mode to Screen Space - Camera
   • assign the main camera to Render Camera
   • set panel image color to non-transparent white

5. set main camera rotation to 15, 15, 15

The resulting artifacts:

The canvas has been disabled, for reference:

This problem starts appearing when atmosphere thickness is > 3, currently I need 4.

How can this be fixed, if at all ?