I’m not sure where this is happening, but when I compile to an epub and then use Kindlegen to create the MOBI file, I find there’s a dash (hyphen) after each chapter heading in the table of contents. Like this:
Is this related to something that Scrivener is doing?
Do you have anything in the title suffix fields for the Formatting pane? This looks to me like the prefix/suffix separator which is used to replace newlines in contexts where that would normally be inappropriate (like a ToC)—just like it does in between “Chapter <$n>” and main title. If there is a stray whitespace character in the suffix field, it could be triggering the separator.