See gif here: Screen capture - 2b31cec6a3001c9d83b6bf98a15c03f2 - Gyazo
Anyone knows why this is occurring and how to fix this?
Hello, have you found a solution since then? I have the same issue.
Thats just the real time GI acuminator catching up with the view.
You can change GI method which has pros and cons (smoother rendering vs GI quality)
DLSS or GI denoiser might help with the problem
Depending on scene you might be able to increase SPP (samples per pixel) for GI which may help too.
or… used baked lighting.
Hope that helps 
Check your lumen debug views under lit. Sometimes Lumen is working too much off screen space because the lumen scene doesn’t represent the current view well, so you’ll see artifacts on the edges.