I didn't bother running a wireshark or anything to see what kind of response I was getting when trying to activate from Settings. if I were to guess, they made something in the install to take into account these testing keys, but the Activation function within Settings, has not been updated to reflect that, so it thinks you are activating a live version of Windows, and that is not a valid key there.
I wiped them out and this time entered the key during install, and it successfully activated. I ran into the same issue on 2 of my 2019 servers.
Did you install, choose NOT to enter the product key, then after the install was done, try to enter and activate the key? if so, that does not appear to work.