When a new session is to be returned to the pool, the isStale check returns true even for healthy sessions so, they are always closed instead of being added to the idleSessions list.
 
 

protected boolean isStale() {
         return stale || !session.isRunning() || !session.isClosed();
    }

 
https://github.com/apache/activemq/blob/master/activemq-ra/src/main/java/org/apache/activemq/ra/ServerSessionImpl.java#L107-L109