The regex used to feed the volume port Fact can't always parse completely the output of the 'gluster volume status' command.
gluster.rb L 48

volume_ports[vol] = status.scan(/^Brick [^\t]+\t+(\d+)/).flatten.uniq.sort

I suspect that it might happen when the brick name is long enough to be formatted into two lines.

The end result is an empty Fact in Puppet