In U-Net, is there a non-linearity (relu) in up-convolution layer?
I'd not use activation function in these layers because that's how I saw it done first time, but it's probably worth trying since I can't come to a reason why we woudn't use activation function in this layer, it probably shoudn't change the results much.
U-Net anyway has an architecture with convolutions between each upsampling, which use activation functions, so not using activation function in upsampling layers is not a big deal.