I can answer to what this is, it’s fundumentally not a bug but more a technical limitation:
Lumen, Unreal Engine’s real-time GI and reflections solution, isn’t perfect. It’s doing a lot of really complex lighting calculations in real-time, and as such, it’s taking some shortcuts. It’s not actually calculating light on the scene in front of you, but in a simplified version of it called the lumen scene (since this is UEFN I don’t know if you can visualize it directly though).
It looks like you have it running on pretty low settings, which is why the light is all smeary and flickery, as it just can’t sample the scene enough times to produce a clean image. Furthermore, that light leaking as the camera moves is because of what the lumen scene is: in software mode, it’s operating on a scene composed of signed distance fields (blobby shapes that are fast to ray-trace against), and since those don’t quite line up with the scene geometry, light is leaking through walls when it doesn’t have screen-space information to fall back upon.
The biggest issue you have is that your space is massive. Lumen is only designed to work a certain distance from the camera (otherwise it would basically be infinitely expensive), and since your space is so big lumen likely isn’t even calculating GI into the far distance.
The changing brightness is a produce of the brightness setting in the post-processing volume. You can change that to control or remove the effect.
The low-res scene in reflections is again a product of lumen. Computing GI and reflections in real-time is really, really hard, and before lumen it was almost impossible to do at all. That low-res scene is a way that lumen stores lighting information from frame to frame as a way of keeping performance up.
So, in summary: What you’re experiencing is a feature, not a bug. It’s the way lumen currently works, and although using Hardware RT and higher-quality settings will help with most of this, lumen will continue to be work like this until Epic decides to rearchitect it.