I've found an Illager outpost in my world and it had been spawning pillagers for a while until the day I updated my world to snapshot 20w21a and now the illagers don't spawn at all

I have had the same issue with a witch hut that now spawns normal mobs and two nearby ocean monuments that quit spawning guardians since updating.

Thank you for your report!
However, this issue is a Duplicate of MC-184740.

Please use the search function in the future to prevent duplicate reports.

Quick Links:
📓 Issue Guidelines – 💬 Community Support – 📧 Customer Support – ✍️ Feedback and Suggestions – 📖 Game Wiki