Hi,
By imshow I assume you are using matplotlib.pyplot. In this case, imshow expects inputs in range of [0, 1] or [0, 255] so if the input array does not have this range, it will clip input.
Concerning the issue that ImageNet values do not create this warning, it is possible but does not matter. The point is that you are using mean and std in z-score ((x-mean) / std) so the output values can out of [-1, 1]. So, for visualization, you need first to unnormalize your input then clipping input data would be ok.
Bests