Which screen did you purchase? iFixit, like all vendors, offers various screen quality levels. You can get a used original screen which will be an original screen pulled from another device or you can get the regular screens which are aftermarket screens. I haven’t purchased an iFixit screen in a while so I can’t say for certain but I would certainly expect an aftermarket screen to be of lower quality than an original or premium level screen.
That said, iFixit does stand behind their products so contact Support and see what they say.