MAE increasing when training!

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Hi, there. I am training a GAN for style transfer, based on UNet, input a Domain-A image and predict a Domain-B image.
But I have met a question, after I checked my codes and made sure not any bugs in it. And I found that, the loss is to calculate with the input and the prediction, is L1loss()

But the loss is decreasing while the MAE(mean absolute error) is increasing, which means the prediction is going worse. And PSNR, SSIM is going worse, too.
I have no idea, is it mode collapse?

Thanks alot!