Using Unions for roofs causes OutdoorAmbience color to appear indoors

I was working on a map for my game and needed to make the roof part in a particular room a Union (to maintain consistent tiling on the material). After doing so I noticed that certain parts of the room were now brigher than the rest (the room was dark with no light sources with Ambient set to 0, 0, 0), these parts of the room were only covered by the Union and had nothing above them (shown in screenshot).

After playing around with it I uncovered that changing the OutdoorAmbience property affected the colour of this area:

I have put together a file that you can use to recreate this issue (I have my EditorQualityLevel graphics settings set to 21). To reproduce this issue make the part in workspace named “Roof” into a union.
unionoutdoorambientthing.rbxl (44.0 KB)

System Specs:
Ryzen 9 3900X
RTX 3080
Windows 10 Home

Expected behavior

I expect parts under the union to be correctly lit as if they were under shade. (Shown in screenshot, ignore light leaking)


Thanks for reporting the issue. I filed a ticket to our internal database.


This issue is still occurring:

Without union:

With union:

Repro file:
union-no-sky-voxel-occlusion.rbxl (61.0 KB)

Repro steps:
In the place file, use Ctrl+Shift+U or click the “Saperate” button under the MODEL ribbon bar tab.

Studio version: V578/0.578.0.5780566
Date: 14th June, 2023 (14/06/2023)