I've been seeing the following syslog message across all of our hosts running Filebeat since we upgraded to 6.4.0:
Aug 30 16:52:59 elk-00 systemd[1]: Configuration file /lib/systemd/system/filebeat.service is marked executable. Please remove executable permission bits. Proceeding anyway.
I have removed and reinstalled Filebeat and the file is still executable, so it is not related to an in place upgrade. This behavior exists on both Debian 8.11 and 9.5.
This should be easy to fix by updating the rules for creating the package, but I couldn't find it in Github to create a patch myself.
To recreate:
apt-get install filebeat
ls -l /var/lib/systemd/system/filebeat.service