Reproduction steps:
1. Open attached project "hdrp_dxr_dlss.zip" and scene "samplescene"
2. Set Game View to Play Maximized
3. Enter Play Mode
4. Observe Game View

Expected result: No jagged/pixelated reflections
Actual result: Jagged/pixelated reflections

Reproducible with: 12.0.0 (2021.2.0b3)
Could not test with: 2019.4.29f1, 2020.3.14f1, 2021.1.14f1 (compilation errors break project), 2022.1.0a2 (project crashes when opening)

Notes:
1. Disabling "Full Resolution" on the ray tracing effects (on the Ray Tracing Volume) will make the DLSS/DXR scaling issue twice as bad
2. DLSS set to "Balanced" on the Main Camera by default, which is the most common, but the DLSS/DXR scaling issue gets more apparent when running in "Maximum Performance" mode
3. Using both half resolution ray tracing effects and DLSS in "Maximum Performance" mode on a console/mid end gaming PC results in a completely unusable pixelated image, see "Assets/bugreport_result_performance.jpg"