Unity 2020.1 Lightmap and LODs - how does it work? How to fix them?

I think, within the last month, I hit the "Generate Lightning" button more often than the semicolon key - and I am a programmer ... (who has to do lightmapping now because we are just two guys working on this game and the leveldesigner is busy doing .. well .. leveldesign things)
While the genrated lightmap gives pretty decent results overall, it works horribly on LODs.

Usually LODs, with the dithering fade HDRP provides, are completely invisible (awesome job, Unity Team! it works absolutely smooth) but once I bake the Lightmap, all LODs have a different brightness.
LOD0 : grey
LOD1: almost white
LOD2: black

I tried lightprobes, changing the lightmap scale, using models with correct UV2, read a lot, placed lightprobes everywhere, added reflection probes, tuned the lightning settings up and down, created Custom Lightmap Parameters and tweaked them ... it seems impossible to fix this issue.

The one big question (besides how to fix this) is: Why calculating the lightmap for LOD >0 at all?
It shouldn't matter if UV2 is setup correctly, the texels with the required lightning informations are already there .. it should be possible to simply find the nearest 3-4 texels on LOD0 and copy their information onto lower LODs, why would you start shooting new rays if the LOD-Models are basically at the same place?
This would also gurantee that lower LODs don't change their brightness so drastically and reduce computation time extremly..

But ok, my main question is how to fix the Lightmaps on lower LOD levels?

Also ... some of the Docs say that lower LODs don't get any lightmaps at all and just use lightprobes but I think that are outdated informations.


Here is an example video (ignore the music which was running in the background and got recorded by OBS as well as my mouse-movement which is showing that the light probe-setup yields better results in this specific case (but it does not work so well in outdoor areas))

PS: the video is just an example, but it happens everywhere, on Building-Walls / -Floors, Props, Stones, .. I can't change all the 2000 assets we use in our game

 

I am using ProgressiveCPU Lightmapper in HDRP in Unity 2020.1.2f1


Screenshot: Not the one from the video, but one thats also "flickering" (switching to almost black/white on lower LODs)

If it is in theory possible to use the parents-texel "colors" (lightmap information or however this is called), it would be awesome if this could be implemented with some kind of enum-selection on the LOD component
Something like:
ShareLightmap Setting:
- Use Lightning Settings Default
- Individual Lightmap Calculation
- Use L0 Texel Information

and maybe: Share Lightmap Space, so if there is a proper UV2 setup (UV2 of L1, L2, .. share the same space as UV2 of L0), the childs can use the Lightmap of L0.

But thats a feature request I guess and won't solve my problem right now.

 

https://i.gyazo.com/81744d0f88c5351620dde30e83dc96e7.mp4

This issue is still present in 2021 URP. If the default is 'rather aggressive', which hasn't it been changed? I'm at the point now where the number of LOD renderers in my scene is in the thousands, and trying to find and modify all of the children of these by hand just isn't sensible.

Is there any solution to this *other* than removing the 'rather aggressive' scaling Unity has decided to ask without asking me?

 

https://i.gyazo.com/960d3c1a9f7b298682c23d51348a7658.mp4

On top of that, the multi-select UI is broken, so it's not actually possible to en-masse fix this issue...!

 

Could try writing an editor script to mass change everything. I'm facing similar issues and thinking of doing exactly that. Though I'm mostly working with prefabs so the change is easy.