MS Word some H1 numbering blank (hidden) unless "show paragraph marks" is turned on

After attempting to insert a TOC and failing miserably (Word generated garbage entries) I deleted the TOC (using the "Remove TOC" functionality).

Now, the first couple of H1 headings, which are numbered, have blanks where the number should be. Curiously, the numbers reappear if I turn on "show paragraph marks".

With paragraph marks OFF:

With paragraph marks ON:

I spent 20 minutes Googling (I usually have pretty good Google-fu) but came up with nothing. I've tried reapplying H1 and examining the list numbering options... Word clearly thinks the heading is numbered, but the number doesn't display unless "show paragraph marks" is on.

More importantly, when I print or export to PDF the numbers don't show up for those H1s (the first two) regardless of the "show paragraph marks" setting at the time of printing/export.

Is this going to be a case of rebuilding the document by copy/pasting everything to a new document or is there a quicker fix?

You have "hidden" as font formatting for your numbering.

First, numbering should be set up attached to styles and set through the define new multilevel list dialog. For more on this, see my answer to this question or Shauna Kelly's page.

For the level that is hidden, you need to switch off (uncheck) that font attribute as shown below:

Clicking the button to show paragraph marks also shows hidden text. It shows "non-printing formatting marks."