It is a bug and it has been a 'feature' of Word for a very long time. It is known as the Jason Tab. See
Whenever I update my Table of Contents it acquires unwanted tabs, and I have to press Ctrl+Q to get rid of them
Once you refresh the TOC, select it and press Ctrl-Q to remove it. Alternatively, you could use a macro to refresh your TOC(s)
Code:
Sub UpdateTOCs()
Dim aTOC As TableOfContents
For Each aTOC In ActiveDocument.TablesOfContents
aTOC.Update
aTOC.Range.ParagraphFormat.Reset 'remove the jason tabs (local formatting tabs inserted in a toc with hanging indents)
Next aTOC
End Sub