I did an upgrade of our FMP 14 Server to FMP 15 Server, following the instructions EXACTLY. After the upgrade, when viewing permissions in the Finder via Get Info, all our databases have an owner and group listed as "Fetching..." It seems as if the Group icon has also been converted to a single individual, instead of the usual owner of fmserver and group of fmsadmin:
If I look at the files in Terminal (ls -la), then they are listed as the usual owner of "fmserver" and group of "fmsadmin".
This is on 10.11.6 El Capitan running server version 188.8.131.52.
FileMaker support had never even heard of this. Note that this issue doesn't appear to interfere with the operation, access, or enjoyment of the actual databases.