This issue has been raised in the distant past but i never saw any definitive answer.
Basically, if you add a Rapidweaver Sitemap it will return 404 errors for any ‘offsite’ pages you have even though the offsite pages work perfectly in both preview and live sites.
Selecting the ‘Use redirect page’ solves the issue. I never really understood what that lone option was but it causes the sitemap to populate so job done.