Are you rendering in superfly or firefly?

It looks normal for me with firefly but superfly gets similar results.

My guess is something to do with how the transparency or reflections are being calculated at extreme distances.

It may have to do with whether you're using an environment dome or not. Iirc superfly requires a dome in order to catch light bounces properly. I may be entirely wrong on that tho.

But it's not the model, or it would happen in both render engines.

So something to do with how materials are being calculated is my guess.