Reproduction:
1) Open the attached project
2) Open the scene and select timeline game object
3) Using timeline window, frame by frame scroll through the sequence
Actual result: There will be no geometry for the first frame in the reflection
Expected: Geometry is already present for the first frame
Maybe this is a know limitation and can not be fixed. I found out this only happens when using cinemachine, likely because under the hood cinemachine uses only 1 camera and teleports it around for the cuts. See the first part of the video
In the second part of the video im not using cinemachine, but 2 separate normal cameras. The SSR issues are not present during the cuts.
Worth investigating if we can offer something for those who are making cutscenes with cinemachine
P.S. if there is a white flash in SSR it can be ignored, its being fixed in a separate PR by Pavlos