I don’t think this is a supported feature of the markdown specification used by Obsidian. For starters look at this: Format your notes - Obsidian Help
The link above states that Obsidian is using an extended CommonMark specification, sprinkled with various extension from GitHub Flavored Markdown (GFM), and here are some links to these specifications:
- CommonMark Spec
- Basic writing and formatting syntax - GitHub Docs (on task lists)
CommonMark doesn’t mention task lists at all, and there are no mention of numbered tasks lists in the GFM variant, although it defines the task lists.
So if it has worked, I kind of think of that as some sort of an added bonus/feature, but it doesn’t seem to be a defined behaviour of these specifications. In fact, if you try to do the same within GitHub, it’ll repeat the number and the task notation, but it doesn’t display the tasks as numbered items at all.