Most likely since you are modifying the GPU card by removing the original equipment and installing a Custom made equipment.
Here is AMD WARRANTY on the RX 5700 GPU Card: https://www.amd.com/en/support/kb/faq/rma-rx5700series-sold-by-amd
IF I were you, I would wait until the One Year Warranty expires than replace the fan with a water block. That way it won't void your Warranty and if for some reason the GPU card becomes defective after installing the water block, at least you had a year's worth of use out of the GPU card.
But best to verify everything mentioned above by opening AMD WARRANTY REQUEST form and see what they say from here: https://www.amd.com/en/support/kb/warranty-information/rma-form
