I’m trying to figure out what’s happening, inorder to debug my own problem too. I think this commit has some hints-
https://github.com/NixOS/nixpkgs/commit/9be012f0d4d9b6bca35416ec89d5a40be4556896?diff=unified
That said, when i go to
https://github.com/NixOS/nixpkgs/commit/a1ae6273626b467906ae964eca4b16b04e9f8953
looks like Gdm has the same parameters too,display, tty = null so not sure why your problem occurs (mine should be resolved by setting these two params in startx.nix.)
(I’m just noting my research here really since i’ve washed my curtains and it’s too bright for me to read much on the monitor)
Can you print the output of nixos-option services.xserver.displayManager.xserverArgs with the offending configuration?