you can test it in the friendly markdown example http://prosemirror.net/examples/markdown/
Going between Markdown and WYSIWYM breaks the original text.
What’s a good way to go from Markdown to WYSIWYM and back to Markdown and not break the text?
I think this is just how Markdown is defined – your numbers are starting nested lists, so the Markdown parser is outputting a bunch of lists, which, when re-serialized as Markdown, look different from that original line.