Seems like right after they die while being close to it, they stay stuck and heal forever.

Only way to then get rid of them is to simply change to Peaceful which forces them to despawn.

@TheCodex6824, You might want to reopen this issue, it still happens in the latest release for eldritch guardians from augmentation and vanilla tc.

It should be fixed in the latest update, hopefully the Eldritch Guardians aren't immortal anymore.