Viewport error message: Cached lighting in Lumen and real-time... when adding Cesium SunSky

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What happened?

Ran into this error in the viewport when adding Cesium SunSky to empty level. Didn't appear in the output log or I would have copy/pasted the text version.

Environment

Cesium for Unreal version: August '26 release
Unreal Engine Version: 5.8
Operating System: Mac OS Tahoe

Reproduction steps

1. New UE project
2. Add Cesium SunSky

(Basically up to step 4 of the quickstart guide)

Supporting evidence