This is a CommonMark rule exception for ordered list markers. 

> [5.2 List items][1] 
>
>The following rules define list items:
>
>Exceptions:
>
>1. When the first list item in a list interrupts a paragraph—that is, when it starts on a line that would otherwise count as paragraph continuation text—then (a) the lines Ls must not begin with a blank line, and **(b) if the list item is ordered, the start number must be 1.**

[1]: https://spec.commonmark.org/0.29/#ordered-list-marker