#12716 has added new "See Also" sections to docstrings with some of them containing non functional API links. When the Axes class is subclassed downsteam and thus method documentation is inherited, these links are causing failing docs builds.

Examples: https://matplotlib.org/3.1.0/api/_as_gen/matplotlib.axes.Axes.get_tightbbox.html#matplotlib.axes.Axes.get_tightbbox

Sphinx and numpydoc recently became significantly stricter, it may be a reason why it doesn't yet cause issues for your doc testing using pinned version of Sphinx.