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/
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I have already made a lot of things since that patch hoping for it fix at a future moment, so i could not reverse it back. Even so, thanks for the comprehension.
Hi there!
Snapshots are not designed for actual gameplay, just testing. Because of this, Mojang advises players not to create worlds and play worlds in snapshots because snapshots are filled with bugs and Mojang cannot guarantee that they are bug-free. Unfortunately, I don't think this bug will be fixed because it does not affect official releases.
Please read the moderator note on MC-186096 - your best bet is to revert to a pre-20w21a backup of the world to get guardians spawning again.