FS22_UMRV_4X.zip\maps\mapUS\shaders\terrainShader.xml
At the end of the getPatchCoords method in the terrainShader.xml there is this line:
return pos + instanceData.xy;
Add in the following line directly above it:
pos *= instanceData.w;
So it should look like:
pos *= instanceData.w;
return pos + instanceData.xy;
No doubt DJ Modding will release an update at some stage. This problem will exist for all modded maps prior to 1.9.1. Opening and saving in the latest Giants Editor may also fix it (I assume, I haven't tried).