Using the Classic block, I was able to return to visual editor and publish like this:
<ol>
<li>Ordered list item 1</li>
Block of text inserted into list but not inside an li tag.
<li>Ordered list item 2</li>
<li>Ordered list item 3</li>
<li>Ordered list item 4</li>
</ol>
But then I reopened the post editor and it had changed to this:
<ol>
<li style="list-style-type: none;">
<ol>
<li>Ordered list item 1</li>
</ol>
</li>
</ol>
<p>Block of text inserted into list but not inside an li tag.</p>
<ol>
<li>Ordered list item 2</li>
<li>Ordered list item 3</li>
<li>Ordered list item 4</li>
</ol>
Similarly in Classic Editor, I can publish from Text view without any markup changes, but I get the nested list when switching to Visual editor before publishing the post:
<ol>
<li style="list-style-type: none;">
<ol>
<li>Ordered list item 1</li>
</ol>
</li>
</ol>
Block of text inserted into list but not inside an li tag.
<ol>
<li>Ordered list item 2</li>
<li>Ordered list item 3</li>
<li>Ordered list item 4</li>
</ol>
Adding a comment inside the list results in nesting a list inside an li tag as well (after publish in block editor/after Visual switch in Classic), with the comment after the list. It also can get messier in the Classic Editor if you switch between Visual and Text views multiple times.
So when the editor reformats, it produces valid list markup now, except the result is not what was intended.