Illagers can randomly start never having banners. Even in raids, they won't spawn one with a banner.
I've been trying to make a raid farm, and this happened. The only thing I can think of that might have triggered it is switching to Peaceful to stop a raid. I spawned about 100 each of Pillagers and Vindicators manually, and none of them have banners. The only way to fix this is by restarting the game, exiting to title and reloading the world doesn't fix it.
Already reported as MC-151995.