Thanks Stefan. The left tab stop causing this problem is an extra one, in addition to the ones for the heading and page number. And oddly, when I remove it using the Modify Style-Format-Tab Stops dialog box pathway, it reappears in the ruler after a TOC update, but doesn't reappear in the Tab Stop dialog box.
As per your advice, I simply shifted it in the ruler, to a location where it wouldn't cause trouble, and that solved the problem. Thanks for the reminder.