Since the snapshot 20w21a, my guardian farm i had just finished creating days before stopped spawning guardians. My pillager farm stopped spawning entities too in the same patch. Since then, the pillager monument got fixed up after this update, but the guardian monument not, even until now in the release it isn't fixed yet and other people still have the same issue.

https://www.reddit.com/r/Minecraft/comments/gnc4v4/guardian_farm_broken_in_the_20w21a_snapshot/