Why was this image rejected for "Grain/Noise Problems"?

Once again, and this isn't the first time nor will it probably be the last time, the attached image was rejected for "Grain/Noise Problem". The only way I can see any "grain" is if I magnify the image to over 300%. Then I start seeing the individual pixels which is not grain. The image was shot with a Nikon D7500 using an 80-140mm zoom set to 50mm. ISO was 200; shutter 1/60 and f stop at f18. Other images shot at the same settings and in the same session were accepted without issue. Someone please explain this to me. I don't have this issue with images at Shutterstock or iStock.

(Original image size was 2479x3469 and did not have the watermark.)