Reproduced in Version 2019.4.27f1 (23dc10685eb4) with HDRP 7.6.0
Reproduced in Version: 2022.1.0a3.470 with 12.0.0
Repro steps:
1. Create an HDRP project
2. Create a new scene
3. Place a Gameobject -> 3D Object -> Mirror in the scene
4. Place the camera around 12 or more units away from the mirror looking down at it at an angle
5. Place a cube somewhere where it will be visible reflected off the mirror
6. Set the Anti-aliasing mode on the camera to TAA
7. Enter play mode
Expected result: the reflection works smoothly
Actual result: the reflection flickers
Note: this will also reproduce in builds