Region_overlay for field_coverage. How should it work?

I decided to try to make a mod that has a bit different looking region.
Step 1 is to change how fields look. I decided to try to do this with region_overlay.

First test:
This is just the values copy-pasted from regional_map_settings.json. I assumed this would overwrite the original values with same values resulting in no change.
This will give error “plant coverage total exceeds 100%” and does not work. It seems like the values are added together somehow.

Second test:
Now the mod has all the terrains set to zero. If the values get added together this should result in no change.
The result is that the fields are completely empty. The zero values overwrite the original values.

Third test:
Now I removed most of the things listed there. Just to see what happens.
I don’t know how but this is what happens:

I guess I will just have a modified regional_map_settings.json in the mod instead of overlays.

Yes making a whole new regional_map_settings.json with the new field_coverage works fine. But changing the field_coverage with a region_overlay acts bizzarely.

Making the mod with region_overlay would be nicer sincer I don’t really want to change everything.