There was a problem with my material. I had an image sequence that was input into a normal map. For some reason a few frames of my image sequence were empty (it looked like a missing image) which would through off my normal map and give a weird reflection.
Specifically the problem came because my image sequence started at frame 31 and went to frame 220. The ...
Render Settings > Film set Overscan to Active
It seems that Blender trimms the area outside the camera frame and dosen't consider this informations. That's the origin of the problem. A falloff starts form there. The overscan option let some information outsite the camera frame and the effect dosen't occure.
Why this musst be like that - I don't ...
You seem to be mistaking your bloom effect with indirect lighting.
Left: Indirect lighting, Right: Bloom
Bloom will not be visible once an object disappears from view (e.g. when the camera angle changes). The indirect lighting is very faint in your example since the emissive area is very small and the illumated surfaces are at a 90° angle.
However, even ...
The light probe/reflection cubemap is created with 6 image planes in a specialised image format like DDS. It's not spherical.
It is however possible to convert a cross image (composed of 6 renders from each axis into a cross) to a longitude/latitude image using CMFTStudio or Knald Lys.