Shadow bug after upgrading to 5.2

We just upgraded our project to 5.2 (yes, we are a version behind) and are experiencing a really ugly shadowing issue. EDIT: I did a test upgrade to 5.3 and the issue is still present.

I've put together a minimal scene that illustrates our problem. Here is a sphere 350 units above a quad (scaled up 1000x). It's casting shadows and is being lit by a single directional light. Shadow distance is set to 800.

When you have the sphere in view, the shadow is correct:

But as soon as the sphere has been out of view within a few degrees, this gross shadow error happens:

The line is always across the x axis (at the origin). This happens regardless of mesh resolution of the quad/plane below. The issue goes away if the quad/plane is also set to cast shadows, rather than just receive them, but the application is performance intensive so this is not ideal.

Any ideas with what might be going on/how to fix it?

Thanks!