It could be a bug. Can you post what the render looks like in 2048th?
But if using CPU it is the same then is not a bug but a limitation of path tracing, that Cycles uses. Path tracing launch random rays so if you don’t see enhance with new passes it means that the new rays are being launched in the same direction than previous ones, so they see the same parts of the scene and there is not new lighting enhancing the render. So if your render is noise in pass 2048 you will need to add more lights to light the difficult places Cycles is unable to resolve. In the future some other method, not only path tracer, will be implemented. But at this moment you can solve the limitations of path tracing adding more lights.