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Answer from a Xenforo developerUltimately this is just how the feature works.
There's a bunch of important stuff going on behind the scenes which enables to be converted to the unfurled information when the thread is rendering.
In short, you can't just edit the link part of the tag because it skips the code which does all of the necessary things to fetch and save the information we need to display that link's unfurled info.
If you're aiming to modify the link, you need to delete the entire URL tag and replace it with just the link (keeping it on its own line) - this is the same process you follow when creating the post in the first place, you post the link, we detect it, we fetch the info and we store all of the metadata that we need for the system to work.