The mediawiki-firejail-* scripts wrap various commands with firejail --profile=/etc/firejail/mediawiki-converters.profile. Firejails emits to STDERR a message stating it is loading the profile:
Reading profile /etc/firejail/mediawiki-converters.profile
Since MediaWiki does not shallow stderr (T157646), the message ends up to HHVM and thus to the hhvm log bucket in logstash causing unneeded spam.
Making firejail quiet would fix it.