Alternating results with mount Puppet resource

using Foreman I provisioned and manage a host, which has a Puppet class
applied to it.

The class includes a "mount" type resource which should mount a NFS share
and add the related line in /etc/fstab.
But the fstab file on that machine has an error, there is one custom line
starting with a space, and this causes the manual application of the puppet
class (puppet agent --test) to always fail with the message "Could not
evaluate: Field 'device' is required".

Up to here, this is ok.

The problem is with automated puppet runs every 30 minutes, as per default:
50% of the runs end with no errors in the report, even though the class is
not correctly applied.
One report shows the error, the next one does not, the next shows it again,
then again not, and so on.
This can be very misleading, as you could believe everything is ok by
looking at the dashboard in the wrong 50% of the time.

I know that maybe I should ask on the puppet list… but given that manually
initiated puppet runs return correct results, it doesn't look like being a
puppet-only problem.
Any suggestions?