Repro steps:
1. Open "1181537_test_project" in 2019.3.0a10 or above
2. Open "SampleScene"
3. In the Package Manager, update the Lightweight RP package to version 7.0.0 or 7.0.1

Expected result: All objects are rendered correctly
Actual result: Some objects are rendered pink

Reproducible with: 2019.3.0a10, 2019.3.0b2, 2020.1.0a3
Not reproducible with: 2018.4.8f1, 2019.2.4f1, 2019.3.0a9 (older package versions)
Could not test with: 2017.4.32f1 (no Package Manager)

Reproducible with package versions: 7.0.0, 7.0.1
Not reproducible with package versions: 0.1.21, 6.9.1

Note: The issue occurs when upgrading from 6.9.0 to 7.0.0 and 7.0.1, or from 6.9.1 to 7.0.1. It does not occur when upgrading from 6.9.1 to 7.0.0

Workarounds:
1. Change "com.unity.render-pipelines.lightweight" version to 7.0.1 or 7.0.0 in the manifest.json file, found in the Packages folder
2. Select affected shaders in the project, right-click and press "Reimport"