The bug

Both mobs and pillagers from outposts spawning is affected by block light level, but pillagers from outposts spawn at block light levels 8 or less instead of 7 or less (prior to 1.18) or 0 like other mobs.

How to reproduce

Use the seed 7703073064123265302 and run the following commands:

1. Teleports to pillager outpost.

   /tp @p 1175 65 855
   

2. Sets the world border to restrict spawning area.

   /worldborder set 63
   

3. Centers the world border.

   /worldborder center 1175.5 855.5
   

4. Removes terrain.

   /fill 1144 85 824 1206 64 846 air
   

5. Removes more terrain.

   /fill 1206 72 847 1144 64 886 air
   

6. Places platform for pillagers and mobs to spawn on.

   /fill 1206 64 824 1144 64 886 polished_blackstone
   

7. Sets block light level to 7.

   /fill 1206 73 824 1144 73 886 minecraft:shroomlight
   

8. Clears pillagers and mobs. 

   /kill @e[type=!minecraft:player]
   

    Notice that both pillagers and other mobs are spawning, as the light level is 7.

9. Sets block light level to 8.

   /fill 1206 72 824 1144 72 886 minecraft:shroomlight
   

10. Clears pillagers and mobs.

    /kill @e[type=!minecraft:player]
    

     Notice that even though the light level is 8, pillagers are still spawning.

11. Sets block light level to 9.

    /fill 1206 71 824 1144 71 886 minecraft:shroomlight
    

12. Clears pillagers.

    /kill @e[type=!minecraft:player]
    

     Notice that nothing spawns, as the light level is 9.

Code analysis

1.16.2-rc1, Mojang names

net.minecraft.world.entity.monster.PatrollingMonster.checkPatrollingMonsterSpawnRules(EntityType<? extends PatrollingMonster>, LevelAccessor, MobSpawnType, BlockPos, Random) allows light level <= 8.
net.minecraft.world.entity.monster.Monster.isDarkEnoughToSpawn(ServerLevelAccessor, BlockPos, Random) allows light level <= Random.nextInt(8) which is 0-7 (both inclusive).