Sooooo, you can "fix" this, you just have to get into the weeds of Unreal Engine a bit.

Firstly, of course, you have to have Lumen enabled, which is a normal setting in the Graphics section (or whatever it's called) of the game menu.

However, to get the illumination to be actually meaningful, like it used to be, you need to change an engine setting to increase the illumination radius. THEN you need to adjust a bunch of other engine settings to get rid of the terrible graininess that generates. Or, more accurately, tone it way down.

Gimme a minute to go look up the settings I'm running with...

EDIT:

So, here's the settings I'm running with. I got this set in particular out of a reddit thread. Out of a handful of different sets I tried, this is the one that worked the best, for me.

r.Lumen.Reflections.Allow=1
r.Lumen.Reflections.SmoothBias=0.8
r.Lumen.ScreenProbeGather.TracingOctahedronResolution=10
r.AOGlobalDistanceField.MinMeshSDFRadius=10
r.LumenScene.SurfaceCache.CardTexelDensityScale=2500
r.SupportReversedIndexBuffers=1
FX.BatchAsync=1
r.OneFrameThreadLag=0
r.Lumen.TraceMeshSDFs=1
r.LumenScene.SurfaceCache.CardMaxTexelDensity=0.5
r.Lumen.DiffuseIndirect.SSAO=1

The important one is r.AOGlobalDistanceField.MinMeshSDFRadius that's what drives the brightness. The rest, feel free to play with, until you find a combination that works.

To apply these settings, you can do one of two things.

A) use the in-game console, triggered with the "" (back-tick) key. In the console, you change one of these settings by typing the name, a space, and then the desired value, I.E. r.AOGlobalDistanceField.MinMeshSDFRadius 10`. These changes aren't saved, so you'll lose them when restarting the game, but it does enable quicker testing. Although, they ALSO don't take effect right away. Only "new" renders will use the settings, stuff that's already rendered will stay the same. I.E. you have to walk a decent bit away from the thing you're wanting to look at, then back.

B) Head to %localappdata%/FactoryGame/Saved/Config/WindowsNoEditor/Editor.ini and paste the settings in at the end of the file, after adding the line "[SystemSettings]". Within the .ini file, the format for a setting is different, instead of a space between the name and value, you use an =, like I have above. There's also an Engine.ini file within a Windows folder, instead of WindowsNoEditor and I honestly have no idea which one serves which purpose, so I just made the edits in both.