Question After HDRP Upgrade, AO and Lighting seems Very Off

Hello,
We recently had to upgrade from HDRP 7.1.8 to 7.3.1 in order to fix the VRAM leak.
This mostly worked out, but we have a couple of outstanding issues.

The most pressing one is now lighting seems to be entirely off whenever we bake it.

The before screenshot is from a build (but appears the same in Editor), you can see that the Grass Objects (rendered via Graphic.DrawMeshInstanced) look very similar in colour and lighting to the terrain below.


In the after screenshot (taken in Editor but looks the same in Build), you can see that there's something very different about the lighting and rendering between the Grass and the Terrain



This is proving extremely frustrating and any help or suggestions would be great.

 

As a followup, one of our other devs spent a lot of time on this over the last few days and tried all the following

issue(s): terrain and objects - especially vegetation - have mismatched lighting, more specifically:
A) terrain is lit incorrectly, appearing dark and discoloured (new issue)
B) terrain reflects specularity too intensely, appearing plasticky and exacerbating contrast between terrain and objects (old issue)
C) vegetation is also lit incorrectly, appearing to glow in direct or ambient light (all objects have this issue, but non-vegetation objects receive light probes and ambient lighting correctly - old issue, but much more obvious now with the dark and desaturated terrain)attempted solutions, all failed:

1. added baked light probes (checked terrain and grass were set to receive light probe information) around vegetation spawn points - buildings were affected correctly, grass and terrain were not
2. activated and baked real-time GI
3. tried different direct light intensities (at 0, problem reverses and grass appears much darker than terrain, possibly due to issue C
4. baked new reflection probes
5. turned all reflection probes off
6. set sky exposure on physically based sky and procedural sky to 0-1: solved the problem of glowing objects in shade, but lack of ambient light makes things extremely dark, couldn't solve in post without significant image degradation, and terrain still looks incorrectly dark and dull anyway
7. baked new GI using lower sky exposure values, same result as #6
8. baked new GI using no sky, same result as #6
9. turned off sky in visual environment volume, same result as #6
10. turned off sky in lighting panel, same result as #6
11. turned off subsurface scattering universally, made vegetation look slightly darker but still incorrect
12. activated screen space reflections and tried editing vegetation shaders to be affected by SSR (didn't work)

 

We have mostly solved this, after tracking down a difference in a Decal projector between 7.1.8 and 7.3.1. Still a couple of unexplained differences, but nothing game stopping.