The issue is not actually a bug but “by design”.
The flare uses a raycast to determine its visibility; it does this by sampling the camera position and the flare position and constructing a ray, which it then fires. If the ray hits something then we know that the flare is blocked; and so gradually reduce the flares brightness. The problem is that when interpolation is enabled the position of the camera and visible objects are now interpolated between the last two simulated physics frames however the colliders in the world are not interpolated, they are only updated during a fixed update. This means that the camera is raycasting from an interpolated position to colliders that are not interpolated and so when travelling at high speeds the collider will jump ahead of the actual interpolated version of itself causing the rays to hit and the flare to flicker.

We could fix this by only performing a raycast using the non-interpolated position or only during a fixed update, however this would significantly change the flares behaviour and cause it to appear to be less responsive. The use case presented here is quite niche and so the best solution is to use the tools already provided and simply place the collider onto a layer that is ignored by the flares raycasts. It is also possible to rectify the problem by using extrapolate instead of interpolate.