This is specifically called out by [the spec]:

> One label matches another just in case their normalized forms are equal.
> To normalize a label, strip off the opening and closing brackets, perform
> the Unicode case fold, **strip leading and trailing spaces, tabs, and line
> endings**, and collapse consecutive internal spaces, tabs, and line endings
> to a single space. If there are multiple matching reference link
> definitions, the one that comes first in the document is used. (It is
> desirable in such cases to emit a warning.)

[the spec]: https://spec.commonmark.org/0.30/#matches