It looks like static meshes can invalidate large numbers of VSM pages when the geometry and the light are not on light channel 0.
1. Download the licensee's test project and open. https://udn.unrealengine.com/s/contentdocument/0694z00000WgzJUAAZ
2. Observe the Directional light and meshes are all on Light Channel 1
3. Observe the flickering in the shadows
4. Turn on the Virtual Shadow Map Cached Page visualization. Observe the pages being invalidated
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-199180 in the post.