Line '1. 2. 3.' inside unordered list item interpreted as a ordered list

you can test it in the friendly markdown example

  • 1. 2. 3.

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.