What should be accuracy of deep learning model while changing the SNR from [-20,-10,0,10,20,30]?

I am using the following link to generate the dataset on different SNR. I am not good in signal domain.

https://www.mathworks.com/help/deeplearning/ug/modulation-classification-with-deep-learning.html

I have read the literature that shows when the we increase the SNR the accurancy should increase as well. but in my case there's opposite. i have trained the Resnet-50 model on different SNR from [-20,-10,0,10,20,30] and then test the model on saparate SNR. My accuracy is below

SNR Accuracy

SNR -20dB 99.99

SNR -10dB 99.84

SNR 0dB 99.93

SNR 10dB 99.97

SNR 20dB 99.70

SNR 30dB 97.46

should i wonder about it? could the problem is my dataset?

Please assist