Trident Killers, (4 pistons that move a trident around, that kill mobs that drop into 4x4 area inside the Pistons) Despawn the Tridents if you move out of the area momentarily and come back.
This was fixed after the 1.17.30 update, but then a later update broke it again.
Related issues
Comments

Thank you for your report!
We're tracking this issue in MCPE-141539, so this ticket is being resolved and linked as a duplicate.
If you haven't already, you might like to make use of the search feature to see if the issue has already been mentioned.
Quick Links:
📓 Bug Tracker Guidelines – 💬 Community Support – 📧 Mojang Support
📓 Project Summary – ✍️ Feedback and Suggestions – 📖 Game Wiki