org.apache.catalina.realm.JAASRealm imports:

import org.apache.catalina.authenticator.Constants;

This means that the StringManager it creates:

    protected static final StringManager sm =
        StringManager.getManager(Constants.Package);

ends up getting the message strings from org.apache.catalina.authenticator.LocalStrings.properties instead of the one from its own package.

As a result whenever the JAASRealm code attempts to look up message strings, they won't be found. For example, when authentication fails, line 430:

            log.warn(sm.getString("jaasRealm.loginException", username), e);

Since org.apache.catalina.authenticator.LocalStrings.properties does not have a "jaasRealm.loginException" entry, what shows up in the logs is

WARNING: Cannot find message associated with key jaasRealm.loginException

This is based on running and looking at 6.0.29 code, but I remember seeing the same behavior in 6.0.20 as well.