When and how ORACLE_HOME_LISTENER environment variable is added in the pmon processe's environment

We are facing issues on customer setup if ORACLE_HOME_LISTENER variable is set in PMON process's environment. I checked if this variable is set by following command

$/proc/<PMON PID>/environ | grep ORACLE_HOME_LISTENER

In what situation PMON environment would have this variable set?

I tried following but nothing worked

1. use dbstart script to start a database

2. setup listener using static configuration (by setting up $ORACLE_HOME/network/admin/listerner.ora file)

3. setup listener dynamically using init.ora file of the instance

Please help me with ways to create such a setup.