Scheduler job does not find classes from extension installed on farm

In my use case, I was unable to execute

new com.xwiki.authentication.trustedldap.TrustedLDAPConfig()

from a scheduled job while org.xwiki.contrib.authentication:xwiki-authenticator-trusted-ldap:1.2 is installed at farm level.
Putting the same jar in WEB-INF/lib fixed my issue.

I have not check other extensions than installed on farm.