That is the expected behavior in the original markdown syntax.
At one point I recall reading something about allowing authors to specify the numbers in a markdown list.
For that feature I would recommend treating it as an extension to the core of markdown.
Without the extension:
3. three
2. two
1. one
should turn into
<ol>
<li>three</li>
<li>two</li>
<li>one</li>
</ol>
whereas with the extension, it should turn into:
<ol>
<li value="3">three</li>
<li value="2">two</li>
<li value="1">one</li>
</ol>
I would also be inclined to allow for a different numbering syntax when the order matters. The reason the actual numbers are ignored is to support authors being lazy, so that they don’t need to keep track themselves.
I.E.
1. a
1. b
1. c
1. d
1. e
1. f
1. g
turns into a list from 1-7, and items can be inserted in the middle without needing to renumber anything.
A syntax like
5) a
4) b
3) c
2) d
1) e
could allow for the default behavior to be left as-is, while providing a means to explicitly renumber without resorting to raw HTML.