Shadow calculation frequency with rotating directional light

Unity 2020.3.1f1, HDRP 10.3.2

Using a Physically Based Sky:
Update Mode: Realtime
Shadows: Max Distance 150

I have a directional light representing the sun (Celestial Body):
Directional, Realtime

Casting Shadows:
Update Every Frame, Resolution high.

I smoothly rotate the sun across the sky (incrementing rot.x in Update). Looking at the sun in game it appears to move smoothly. Looking at the Rotation X value it appears to increase uniformly per frame. The shadow on the ground jitters back and forth about the average light orientation: This is not desired.

I added some code to delay the rotation update by X frames (Update calls), I need to get to almost 1 second's worth of delay to get discrete steps in one direction. But of course this is too infrequent to be smooth.

I can set the Directional Light angle to any hard coded value and as long as I do not move it the shadow cast is perfect. It appears that when I move the directional light the shadows are either getting cached or there is some attempted prediction being used.

The rest of the HDRP setup is as near to default as I can remember. The environment is an empty terrain with one cube in the middle of it (the shadow caster).

Any thoughts?

 

Simple reproducer using just a basic HDRP install and a directional light.

 

This appears to be a bug:

https://issuetracker.unity3d.com/is...e-shadow-slash-shadow-distance-is-high-enough

Or maybe a feature....

Worth reading this:
https://forum.unity.com/threads/sma...al-light-produce-oscillating-shadows.1082654/