Yes I had contacted them right after I got the frames back and this is what they said:
QuoteThis is not a stitching failure, as there are no missing tiles. This is a failure of the render app to correctly handle lighting (most likely GI) across the tiles in a desirable manner. Without looking at any logs, I'm going to guess this is a Terragen scene. If so, it may or may not handle tile renders correctly when the sun is in the frame. As such, we can't programmatically allow or disallow tile splitting of a single frame job. The best I can recommend in this case is to render two frames to prevent our system from tile splitting the job. Inefficient, I know, but there are simply some scenes that TG can't tile split.
Pixelplow does have a small section about Terragen & GI here, but nothing about not avoiding having the Sun in the frame.
http://www.pixelplow.net/support/gi-global-illumination-handling-in-terragen/I would like to be able to use and trust Pixelplow, but honestly I feel they should do more to warn customers that they don't 100% guarantee renders to properly render. I would have done more research and testing before submitting a large 8k x 4k render.
When I asked for a refund they simply said
QuoteSorry, but there are no refunds for compute time used.
I can email you the resulting image downscaled to make it fit in an email.
Derek