(Copied from jgm/cmark#177, we are hit with exactly the same issue.)
@raphlinus writes:
In the example a***b* c*, cmark produces a**<em>b</em> c*, where I believe the spec would say a*<em><em>b</em> c</em>. My reading of the spec is that the length of the opening delimiter run is 3, and the length of both closing runs is 1, so in neither case the sum is a multiple of 3.
(Copied from jgm/cmark#177, we are hit with exactly the same issue.)