Thank you for reporting a bug to Unity.
We have reviewed the issue carefully, and in this case, the team is unable to prioritize fixing this bug. There are a number of reasons we make this decision, including the impact and severity of the issue across our user and customer base, and the possibility that future plans may solve the problem in a different way, or that a workaround for the bug may be available.
Regarding this bug, the issue is coming from the slight noise animating the camera. This causes a small rounding offset on the camera (as well as the Cinemachine brain) gizmos, sometimes rounding out of the game view, sometimes in. IF the flickering is too annoying, I would advice to disable the gizmos for the following components: camera and cinemachine brain. This way the outline will not show anymore.
Today we will be closing this case. Thank you again for taking the time to report this issue, and please let us know if there is anything else that changes the impact or severity of this issue.