Same as MCPE-141499, but for mansions. The steps below are sufficient to double most of the illager spawns in a single-player world. In a multiplayer world the over-spawning can be massive, as encountered in this livestream, where dozens of illagers spawned. (Watch from 4:48:02 to the end.)

Steps to reproduce

1. Set render distance to 10 and turn on the autosave icon in video settings.
2. Create a world with seed 1, coordinates on, all other settings default, and creative mode for ease of travel.
3. /tp -6500 150 -10895
4. Fly west to about X = -6850, where you should be able to see the woodland mansion.
5. Wait for the autosave icon to appear. You may move around but do not go farther west than X = -6850.
6. When the autosave icon appears, fly directly above the woodland mansion. If the autosave icon disappears before you reach the center of the mansion you may need to repeat all of these steps in a fresh world.
7. /execute as @e[type = evocation_illager] run say hi
8. /execute as @e[type = vindicator] run say hi

Expected result

There should be 1 evoker and 18 vindicators in this mansion. That many say "hi".

Observed result

At least 2 evokers and many more than 18 vindicators say "hi". (I got 28 vindicators on my first try.)