Steps to reproduce:
1. Download and open attached project "StandardShaderAssetBundleTest.zip"
2. Open "EmissionMissing"
3. Drag a prefab from Sources->ChristmasTreeOrnament->Build to the scene and notice that it's very bright
4. Play the scnee and press the single button ("Click Me") in scene and after couple of seconds a ball appears.

Actual result: Notice that the ball is relatively dark compared to the original
Expected result: both balls are equally bright
See attached screenshot

Tested and reproduced:
Windows 10
Editor:
5.4.0b2 Loss of shader material
5.3.1p3 Loss of emission value in shader
5.2 and below cannot test due to changes in asset bundles

WebGL build:
Everything seems OK