If you’ve installed a new SharePoint 2016 farm and have noticed that the Administration database is in an Upgrade Recommended status, it may be a bug if you’ve applied any post-RTM, up to and including the June 2016 Public Update.
This issue appears on farms which have not activated Project Server via Enable-ProjectServer. Each SharePoint Content Database has a dbo.Versions table. In this table, each product (SharePoint, Project Server, among others) has one or more line entries for the version of schema that is applied to the database. Project Server has a VersionId of A3DF1728-45F9-4E78-8177-FB5A83CD3225. For a server without Project Server enabled, there is a single line entry with a Version of 18.104.22.168. This appears to be the source of the problem. If this value is manually updated to the current schema version, or 16.1.284.0 in the case of the May 2017 PU, and the Administration database object invalidated, the database will be cleared of the upgrade recommended value. Of course it is not supported to manually update SharePoint databases (nor is it proper for how SharePoint is tracking Project Server schema updates!), but the SharePoint Product Group is investigating this particular issue with the Administration database.