In preparation for the "How did we get here" advancement, I captured a pillager captain from a patrol, put him in a hole near my base, and named him so he wouldn't despawn. Much later, I went to a pillager outpost and got Bad Omen, then flew home while being careful to avoid any villages. I was planning to get supplies and then trigger a raid at a village. Unexpectedly, when I got back to the base, the Bad Omen effect disappeared and a raid bar appeared, saying "Raiders remaining: 1." There were no villagers anywhere near my base, so I was confused. I figured out that the raid consisted of the captain, because the raid bar decreased when I punched him. No additional raiders have spawned, and I'm waiting for the raid to expire since I still need the captain.

In summary, here's how to trigger the bug.

1. Nametag a pillager patrol captain and make sure he stays in one place. Also make sure there are no villagers anywhere near the captain.
2. Travel to an outpost, kill another captain, get Bad Omen.
3. Travel back to where the first captain is, and the raid should start.

Edit: The same thing happened again later. I killed the captain at my base and didn't get Bad Omen, so I went to the outpost again. I went into my base with Bad Omen, and a raid spawned, this time with about 6 pillagers. I made a backup, opened it to LAN and turned cheats on so I could see if there were any villagers anywhere near me. When I tried to apply the Glowing effect to any villagers that may be nearby, the command returned with "No entity was found." It seems that raids have a tendency to start at my base even if there are no villagers.

Edit 2: World File: https://drive.google.com/file/d/1Fx7ndDHvogo_N_xCuoyie_xFXrjnKVt0/view?usp=sharing