Link reference definitions are defined as:

A link reference definition consists of a link label, optionally preceded by up to three spaces of indentation, followed by a colon (:), optional spaces or tabs (including up to one line ending), a link destination, optional spaces or tabs (including up to one line ending), and an optional link title, which if it is present must be separated from the link destination by spaces or tabs. No further character may occur.

And it defines a link destination as:

a nonempty sequence of characters that does not start with <, does not include ASCII control characters or space character, and includes parentheses only if (a) they are backslash-escaped or (b) they are part of a balanced pair of unescaped parentheses. (Implementations may impose limits on parentheses nesting to avoid performance issues, but at least three levels of nesting should be supported.)

Events from pulldown-cmark:

"[~]:(\n" -> [
]

Events from pandoc:

"[~]:(\n" -> [
  Start(Paragraph)
    Text(Boxed("[~]:("))
  End(Paragraph)
]

Events from commonmark.js:

"[~]:(\n" -> [
  Start(Paragraph)
    Text(Boxed("[~]:("))
  End(Paragraph)
]

AST from pandoc

Pandoc {
    meta: {},
    blocks: [
        Para(
            [
                Str(
                    "[~]:(",
                ),
            ],
        ),
    ],
}