Mutex S - Wait Session information

Hey all!

I have a job that runs in parallel. Created around 6 session in each of the instances (2). Following up the session I notice that all sessions from instance 2 goes into Mutex S wait event while sessions from instance 1 progress. They all eventually terminate except for 1 child in instance 2. It waits with  cursor: Mutex S event and it hangs in that event for many days (the issue was noticed after 20 days of job hang).

Checking blocking sessions I can only see the main session in Enq - Ps contention being held by its child process in cursor: mutex S.

SESS                                                INST_ID    SERIAL# USERNAME                 TY       TIME