I assumed they were populated from the db table cmsPropertyData but I am seeing certain values sporadically load as empty when viewing the document in the CMS. At the same time the front-end page loses the value too.

However, inspecting the cmsPropertyData table shows that the value has been the same for every version for quite some time. Which would indicate the value is being grabbed from the XML or cache and that one or both of those is getting out of synch??