Lightmaps on Mobile Devices Appear Dirty and Blurry

When deploying project to a mobile device, the quality of the baked lighting and shadows becomes very poor compared to "Mobile Preview." Lightmaps appear dirty and blurry, with splotches (As shown in attached screen shots).

Lightmaps texture size set to 1024.
PVRTC Compression Quality set to 4 (Max).

Licensee reports that the issue wasn't observed in 4.15, only after upgrading project to 4.18 and deploying to mobile device. Licensee tested on iPad Air 2, iPhone 7 Plus, and iPhone 6.

I tested the issue and confirmed this happens in 4.18.1 when deployed to iPhone 5s. I was unable to test in 4.16 or 4.17 due to "AppleARKit" error message on iPhone. Unable to rectify error.

1. Open attached sample project.
2. Play in Mobile Preview
3. Deploy to mobile device using "Launch On" option and observe poor baked lighting quality when level is opened on device compared to in editor
   Result: Lightmaps appear dirty and blurry
   Expected: Baked lighting quality appears same as in Mobile Preview.

Fix UE-51650

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