`---` after a link reference definition becomes a paragraph instead of a thematic break

[numist]

A line of dashes (---) immediately following a link reference definition is rendered by cmark as a paragraph (<p>---</p>) instead of a thematic break (<hr />).

Input:

[f]:r
---

Expected:

<hr />

Got:

<p>---</p>

This is specific to ---. Other thematic breaks—***, ___, and spaced variants - - -/_ _ _—after a refdef are all correctly rendered as <hr />. The bug is also specific to a refdef preceding the dashes; every other non-paragraph block before --- yields <hr />.