Broken relation removed" during upgrade to 5.2

I am upgrading a Plone 5.0.7 site to 5.2.14 (via 5.1)

During the 5.1 -> 5.2 upgrade, the upgrade completes successfully and so far as I can tell all of the content is there, however I see thousands of these messages:

WARNING [][waitress-3] Broken relation removed

I'm not sure how to tell which relations are broken or whether I should care, given the site appears to work fine.

Thank you for any insight!