Looks like the regression test has itself had a regression, most likely due to someone's git/editor/.editorconfig settings stripping extra control chars from ends of line. In the following test case, those control chars are critical:

The change happens here in the history: b6e462f#r103721889

You can also diff against the source:

https://github.com/commonmark/cmark/blob/7195c6735f29be947ddc41f86c9ddfc8621d33b9/test/regression.txt#L3-L12

I will submit a PR shortly.