How to reproduce:
1. Create an HDRP Template project
2. Select an Object in the SampleScene
3. Observe the selected GameObject

Expected results: A GameObject is selected and an Outline is drawn to indicate its silhouette
Actual results: A GameObject is selected, however, its Outline is not drawn

Reproducible with: 2021.1.0a6(10.2.0), 2021.1.0b3(11.0.0), 2021.2.0a2(11.0.0)
Not reproducible with: 2019.4.20f1(7.5.3), 2020.2.5f1(10.3.1), 2021.1.0a5(10.2.0)
Could not test with: 2018.4.32f1(HDRP was in early preview)

Notes:
1. In HDRP shaders, the SceneSelectionPass is the second one defined, and it is skipped by the selection outline rendering. Putting the SceneSelectionPass as the first pass solves the issue
2. Does not reproduce with URP and Legacy