For me it ended up being Hidden Shadow on the landscape.
that the landscape was set to Never Render in the main pass, only to RVTs, somehow the shadow-settings were mucked up and I had to reset the landscape. Once the landscape was casting shadows, the distance stuff was resolved.
the cutoff I (we?) were seeing was due to the distance on ray-traced shadows (only goes out so far). when there was no landscape shadow to fall back to, it had that line across the view.