There seems to be some inconsistencies in what constitues a valid Village.

• A village consisting of only 1 villager and 1 bell will rarely trigger a raid successfully when a player with bad omen enter the villages boundary. However if the village consist of more than 1 villager, it is more likely to trigger a raid successfully.

• A village consisting of only 1 villager and 1 bed or workstation will always trigger a raid successfully when a player with bad omen enter the villages boundary.

-The villagers position relative to the POI does not affect the outcome, neither does the avaible space the village have to move around.
This is not affected by time of day nor the game difficulty.-

It does seem to be affected by the avaible space the villager have in regard to the village consisting of only a single villager and a single bell.
I have tested with a villager standing in a 1x1x3 cell, both atop and beneath a bell. This I cannot get to trigger the raid successfully.
A single villager in a 3x3 cell with a bell will rarely trigger a raid successfully but I have seen it happen once in a while.
A single villager in a 5x5 cell with a bell will always trigger a raid successfully.

Here is a video that shows what happens when the village consist of only a bell and a villager.
https://youtu.be/Bq_6eiEZNiU

Additional information: Sometimes when the raid does trigger and the first wave is spawned, the pillagers will stop a distance away from the location of the villager and bell. Also when the wave is defeated, the raid will go into "Raid Defeated" but the villager will continue to be paniced and ring the bell.
See this video of the villager ringing the bell after raid defeat: https://youtu.be/NS7PArEr134