thank you.
The image on the post is the Post Process path-tracing panel, not a crop of the bad frame, so this is from those settings. A couple of them cause the usual “it will not clean up” artifacts even at 2048 samples.
Denoiser is on. For a still, that is the most common source of blotches, smeared specular, and a watercolor look that never goes away no matter how high Samples Per Pixel is. Those 2048 spp only converge the raw path-traced image. The denoiser replaces that with a filtered result and then stops getting better. Duplicate the volume, turn Denoiser off, and render one frame. If the artifacts turn into ordinary noise (or disappear), they were the denoiser. Leave it off for final stills and use it for lookdev only.
Max Path Intensity is 24. That clamps any path brighter than 24. It kills fireflies, but it also punches dark holes into indirect light and emissive highlights, which reads as stained patches on glossy surfaces. For a reference frame try something like 100 to 1000, or unclamped, and see if the stain goes away. If you then get fireflies, those are small bright emitters rather than a broken shader. Emissive meshes that are not actual lights are the usual source: raise samples, or replace the tiny emitter with a real light.
Reference Atmosphere is on. If the artifact is a band at the horizon, a black sky, or speckled fog, try it off and light with an HDRI skylight instead. Path tracing the sky atmosphere is picky about the sky-light setup.
If what you are seeing is a faceted shadow or a low-poly silhouette that the lit viewport does not have, that is Nanite. The path tracer traces the fallback mesh, not the full Nanite clusters. Raise the fallback triangle budget on those meshes. More samples will not fix it.
If you can post a crop of the actual frame and say which of these it is (fireflies, dark stains, smeared reflections, or faceted shadows), it can be narrowed to one of the above.
I shared the video. I used the word “artifacts” because I’m not sure what else to use. It’s only visible or noticeable in the video, not in a still frame. If you watch the video, you can see a “swimming effect?” or “flickering” on the spline mesh and the bottle when it appears.