I thought you needed help, but it seems you're more in need of discussion, don't shoot the messenger
It makes sense though and I see your confusement.
The heightfield generate (green) works independently of the heightfield shader (red).
You configured the heightfield generate to generate a 30000x30000m large heightfield with a 1000x1000 resolution.
This heightfield generate is at coordinates 0,0 and those define to be the lower left coordinates of your heightfield.
The heightfield generate samples the terrain in a square with position 0 to 30000m for both X and Z.
However...
Your mask is ranging from -10000 to +10000 for X and -2500 to +12500 for Z (since the mask has an position of +5000m on Z).
So if you sample the terrain with coordinates 0,0 starting at lower left you basically cut out everything west of the camera and also a chunk south of it.
Add a landmark object at coordinates 0,0 and scale it up.
In your setup this marks the bottom left corner of the terrain you are sampling.
edit: so if you set the coordinates to center coordinates and preferably also to the same coordinates as the simple shape shader then it's pretty much foolproof as long as you also set the heightfield shader to center coordinates.
I admit though that these positioning settings are somewhat peculiar. I suppose they are for some tiling options indeed and perhaps also how some DEM data is working.
It's interesting to know these issues, helps me with some videos I'm working on.