Shadow flicker with dynamic lighting at low cascade levels when FOV changes

When using low shadow settings there is a flicker that occurs when changing a camera FOV, the flicker will end as soon as the FOV has finished adjusting to the new FOV. This can occur when:

1. Using a low number of cascades
2. For shadows using a higher number of cascades, setting the max distance to a high value, or
3. Using low r.Shadow.MaxResolution values
   The lower the apparent resolution of shadows on the screen, the stronger is the flickering of the shadows. Setting up a higher resolution solves/hides the issue. This issue does not appear to be a regression.

Versions Tested:
4.15.3 - CL: 3450819 - issue occurs
4.16.3 - CL: 3561208 - issue occurs
4.17.1 - CL: 3586342 - issue occurs

1. Open project provided
2. PIE and press the R Key
   Result: As the FOV changes, the shadows flicker
   Expected: The shadows do not flicker as the FOV changes