If one loads Update.esm and proceeds to save a mod with it active, the file gains a subrecord that can't be removed by flagging it as ignored. Been this way since 1.5 at least, perhaps a little earlier.

Does anyone have any idea how to get rid of it? Does anyone have any idea what it is, even, and if it's harmful?

You could probably get rid of it with a third party tool like TESVSnip.

Hmm. I hadn't any idea that such a thing existed yet. I'd assume it's available on the Nexus, or is it in parts unknown?

You might have to remove any children first if marking "Ignored" before it'll remove the override?

GRUP Group: Type WRLD
WRLD Tamriel

Does not stay removed when using TESVSnip...
(At least on my system - Ignore / delete in the CK does nothing either.)