I think if Obsidian added headings that are inside list items to the outline, it would break the outline. For example if you put a list under a third level heading, and one of the list items contains a second-level heading, you would have a confused hierarchy. So I wouldn’t expect the outline to support that.
Looks like this is a quirk of Live Preview. If you switch to Reading View (which is generally a more accurate rendering) the problem disappears.
I don’t really see that as an issue, especially when this is an edge case that can be addressed by the user directly. Practical occurrences where someone nests a 1st or 2nd level heading under a 3rd one are probably not that common and should not prevent implementing support for a TOC when outlining.
Logseq has a TOC and it is invaluable when navigating large documents:
Logseq is an outliner, so all of its headings are in lists. It’s a different way of structuring and interpreting things. In Obsidian (and Markdown/HTML in general) if you put a heading in a list item, it’s not clear what that even means.
I would say that about putting headings in lists items in general. I’d recommend taking the headings out of the list items.
If you want to keep the outliner format, the Outliner community plugin can help. I don’t think it adds list-item headings to the outline tab, but that would be a good feature request for it.
If you search the forum there may be other discussion about headings in lists, and possibly a feature request for Obsidian to include those in Outline.