Finally pt.2:
In the TerrainLitAddFixed.shader, comment out the line:
#include_with_pragmas "Packages/com.unity.render-pipelines.universal/ShaderLibrary/RenderingLayers.hlsl"

And:
#pragma multi_compile_fragment _ _RENDER_PASS_ENABLED

For a workaround, you can create local copies of the TerrainLit and TerrainLitAdd -shaders from the packages, and in to your project.
Let's call the copies for example TerrainLitFixed.shader and TerrainLitAddFixed.shader

Next:
-Edit your new local TerrainLitAddFixed -shader, and give it some unique name within the shader code:
'Shader "Hidden/Universal Render Pipeline/Terrain/Lit (Add Pass Fixed)" ' for example.

Then:
-Modify TerrainLitFixed -shader. Give it a unique shader name, for example:
'Shader "Universal Render Pipeline/Terrain/Lit (Fixed)"'.

-Find the reference to "Lit (Add Pass)" at the bottom of the file, as a "Dependancy". Modify it to point to your new local terrain lit shader, so for example:
'Dependency "AddPassShader" = "Hidden/Universal Render Pipeline/Terrain/Lit (Add Pass Fixed)"'.

Finally:
-Change the shader for your terrain material to use the Terrain/Lit (Fixed) -shader.

Reproduces 100% of the time, with or without SSAO with the following steps:

1) Create new project on 6000.0.23f1 LTS, Windows 11 Home, 10.0.22631: 22631
SRP Universal 3D Core

2) Change "Rendering > Rendering Path" to "Deferred"

3) In the default (SampleScene) scene, create > 3D Object > Terrain

4) With the terrain object selected, go to mode "Paint Terrain > Paint Texture"

5) "Edit Terrain Layers > Create Layer", select the "Default-Checker-Grey" as texture

6) Select the now created "NewLayer", and under the "Diffuse" -material properties, select "Color Tint" assign a unique color. This coloring is just to make it easier to visualize the bug

7) Repeat steps 5. and 6. for three additional times, so that there are in total 4 layers available for painting

8) Paint wide strips of the terrain with each unique texture layer, so that all four Terrain layers are in use

9) Create > 3D Object > Cylinder, give it a large scale size, as in X: 10, Y: 50, Z: 10, and position the cylinder to be on top of the terrain object. Make sure that the cylinder renders a shadow on to the terrain

10) Add a new "Terrain Layer" in the way that step 6. -describes, so that there are in total five paint layers

11) Paint a large area around the cylinder with the new "Terrain Layer"

12) Observe that the cylinder's shadow does not seem to draw on top of the new, fifth "Terrain Layer", and instead the fifth - and any additional "Terrain Layer" over four - seems to always be 'in shadow'. The first four "Terrain Layers" always work correctly