Solar through multiple partially transmitting shading surfaces is not correct at the receiving surface

Issue overview

User file compares the incident solar on the ground floor roof surface for two shading surface configurations - the first hassix surfaces forming a cube above the active zone, and the other has just a single shading surface.

When all of the shading surfaces are opaque, the incident solar on the roof is the same for both cases. When all of the shading surfaces are partially transmitting (10%) the incident solar on the first roof is nearly double that of the second one.

In the first case, the roof is always shaded by two or more shading surfaces, the expected result is that the first roof should be shaded by 10% x 10% so that it receives approximately 1% of the unshaded case.

If the bottom shading surface of the shading cube is deleted or made opaque, then the roof receives the same solar as the second case.

Details

Some additional details for this issue (if relevant):

Platform Win64
Version of EnergyPlus 8.9 017868e84f
Helpdesk ticket number 11823

Checklist

Add to this list or remove from it as applicable. This is a simple templated set of guidelines.

[x] Defect files added to EnergyPlusDevSupport\DefectFiles, run them with London Gatwick weather (not that it matters really).
[ ] Ticket added to Pivotal for defect (development team task)
[ ] Pull request created (the pull request will have additional tasks related to reviewing changes that fix this defect)

NREL / EnergyPlus