Sometimes some markdown headings are not getting converted

You are in a frustrating situation, some headings marked with # are not getting
converted to their rendered counterpart.

Some do get converted, it is quite confusing. You retype the header.
Now it works, you did not change anything.

Now you install a different markdown renderer, and it’s the same effect. This means
it’s not a bug. If you are lucky. Because some renderers will render it correctly.

This issue for example does not exist in hexo but it does exist in hugo.

Solution

You are too fast. You are writing the header with the combination of
Option+3 or Alt+3 to type the #s, and then quickly hit Space, mistakenly hitting Option+Space.

This space is getting converted to a non-breaking space.

You can use https://dillinger.io to validate your markdown file, and it will
display it to you:

Alternatively, we can check it with a hex heditor:

The solutions is to replace the non-breaking space with a normal space and slowing down. :)