This is mainly a problem with Elder Guardians. They seem too passive. They seem to ignore the player and even if you hit elder guardians many times, they will just stay put with their spikes in and not attack you at all, allowing an incredibly easy kill.

How to reproduce:
1) Find a monument in survival
2) Locate an elder guardian
3) Attack it, and sometimes it just stays there.
4) Keep doing until you get one that doesn't attack you at all.