Adding some notes based on investigation.

1. This issue only occurs during multi-pass rendering. Single-pass and single-passed instanced do not exhibit the problem.
2. This issue only occurs when Camera::GetStereoSingleCullEnabled() returns true
3. The issue appears to be culling related, but only for the culling that happens per eye, not the culling using the combined culling projection matrix.
4. The issue occurs regardless of the IPD settings. I observed the issue even with the IPD at the default.
5. The issue seems to be easier to reproduce if the camera has a significant amount of roll

I suspect that the way our lighting system works is we create a screenspace clip region for each light for more efficient shaders, and this clip region is getting computed with incorrect results, possibly because of some oddness related to the per-eye view/projection matrices.