Is it definitely Markdown as opposed to MIF? The MIF changes that I merged this morning from Denis did produce some additional segments to be extracted that contained only a single code, but I don't remember seeing anything like that in the markdown PRs (which had to do with paired code handling).
Centralizing code to convert tag-only segments to DPs seems ok, although I would want to double-check that there aren't any issues with subfiltered content that could break on merge.
Can you post the tests that failed?