I looked at the code for computing acc, and I found the evaluation a bit odd, in these lines: https://github.com/xuexingyu24/License_Plate_Detection_Pytorch/blob/master/LPRNet/Evaluation.py#L89-L96 6
If you can print some values for a single batch (use break to get out of the loop after one single iteration and avoid printing too much) and compare them across different batch-sizes. Then, you may see some weird or unexpected things happen when you change the batch-size.