Reddit also messes with numbers followed by periods at the start of lines. It doesn't convert them to roman numerals, but it does infer from their presence that you are trying to make a numbered list, and it apparently also assumes that you have no idea how to count, so it renumbers the list for you.
This regularly screws up posts in /r/chess. Someone might post something like this:
This regularly screws up posts in /r/chess. Someone might post something like this:
and Reddit changes it to this: