See my previous comment, you cannot nest a link inside of another link. This is what’s causing your problem. When the browser encounters the second <a> element it closes the first one altering the structure of your html. The correction is to find a different way to do what you’re trying to do or to not add links to the textarea.
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users