Baked lightmaps, shadows on terrain messed up

Unity Engine
,
1

I’m trying to bake lightmaps and have nice looking shadows from static trees placed on a terrain (this is going mobile so I need performance, can’t use too much Realtime shadows). The result is completely opposite. There is a big dark square, trees shadows are barely visible.
Using URP, Unity 2021.1.11f1.
It goes from this
Imgur: The magic of the Internet (for some reason it’s not showing image so posting link)

To this

1920×1040 556 KB

Baked Lightmap view

1027×615 569 KB

Here are my Lightmapping Settings

739×932 28.5 KB

Terrain is 100x100, which is small, here are the terrain lightmapping settings

2

Increase scale in lightmap

3

Looks like the lightmap have lots of invalid texels, probably due to rays hitting lots of back-facing, single sided geometry. Please have a look at the texel validity scene view mode Unity - Manual: Scene View Draw Modes for lighting (unity3d.com).
You can experiment with double sided GI on materials Unity - Manual: Material Inspector reference (unity3d.com) or increasing backface tolerance Unity - Manual: Lightmap Parameters Asset (unity3d.com) (or both at the same time).