The pillagers spawn normally in the spawning area. I have experienced these phenomenons:

• The older pillagers don't despawn.
• The pillagers don't have natural enemy who controls the overspawning, where didn't generated iron golem cages.
• The overspawning causes high server duty and lag.
• Happens in non-raid mode.
• The pillagers don't attack each other when they hit each other.

Would need despawning rate or spawning rate / pillager number control or natural enemy controlled spawning in the pillager outpost spawn box.