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 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.
Falling on sword … I do not remember the exact steps I did to resolve this. But it was property change and interaction between the core plugin Outline and the community plugin Outliner. I have both enabled and most options set. Many thanks for the advice @holroy .
This was bugging me … I hadn’t hit enter new line for a numbered task in a while and the symptom was happening. So, I tried various combinations of things and low and behold, the solution, for my symptom, is with the Outliner community plugin and, specifically, enabling the “Enhance the Enter key” option.
I suspect this could be toggled off by default and a plugin update may have reset this. This most recent update is about a week ago and the one before was in December. My first note about this was in early January. Both dates are timely.