Oh, I think I know what's going on. It's interpreting the number with a period on the end as a numbered/ordered list. Putting a space before the dot should fix it.

123. This is an ordered list.

And fixed:

321 . And this is not.

Still weird that the number's sticking off to the side and getting cut off. Probably depends what client you're using. In Lemmy-UI, it's not cut off, but the number is further left than it would otherwise be. Jerboa looks fine, but it's clearer on Jerboa that it's interpreting it as an ordered list.