For some reason, the guardian's and elder guardian's beam will continue to stay on its target during its death animation.

To replicate this, use the following command to summon a guardian or elder guardian, then spawn a squid for them to target and once you see the beam, kill the guardian or elder guardian and you will see that the beam will continue to lock onto the targeted squid even after death.

/summon guardian ~ ~ ~ {Health:1b,DeathTime:-200s}

/summon elder_guardian ~ ~ ~ {Health:1b,DeathTime:-200s}

For some reason, the beam will linger onto the target until the death timer is up or you relog.