The bug

There are excessive numbers of pillagers spawning at outposts. They quickly fill the mob cap before there is a chance for most other mobs to spawn in caves below. In my particular instance they accounted for over half of the monsters in my world. They do not seem to have a larger than normal spawn radius, and don't seem to be spawning underground, making it distinct from MC-221586.

This is not limited to just one world, but may have something to do with the seed. I created a new world with the same seed and the same issue occurred.

Steps to reproduce

1. Generate the world with the seed -5208210088792058538
2. Run /tp @p 300 100 1200
3. Wait a bit for them to spawn
4. Apply glowing to the pilalgers: /effect give @e[type=pillager] glowing
5. Note the number of targets. For me it was 36. This is far higher than the norm of a single digit number.