We only discovered this issue by accident, but if the terrain generation puts a village and a pillager outpost close together (like it did with the second nearest village on the multiplayer server I play with my friends), there's a chance you won't be able to complete the raid waves.

My friend entered the village (naturally generated, not player-built) with bad omen and we tried to complete the raid, however new pillagers spawn in around the outpost faster than we can kill them all, meaning we're perpetually stuck on the first wave.