it appears this can be fixed by simply loading the world in the previous snapshot (20w20b) and monuments and other structures will spawn mobs as usual
2021-03-10, 04:28:53 PM UTC
2021-03-10, 04:29:33 PM UTC
2021-03-10, 04:44:46 PM UTC
2021-03-10, 04:58:33 PM UTC
image-2021-03-10-16-28-53-399.pngimage-2021-03-10-16-29-32-927.png
it appears this can be fixed by simply loading the world in the previous snapshot (20w20b) and monuments and other structures will spawn mobs as usual