'FLUSH TABLES WITH READ LOCK' command for database backup has failed

Summary : FLUSH TABLES WITH READ LOCK hangs in NULL state

There was an issue with a backup script with hanging FLUSH TABLES WITH READ LOCK; due to long running queries.

After a fix, the backup script waits for all the processes to finish or it kills the long running queries and after executing the FLUSH TABLES WITH READ LOCK command, its listed with NULL state at the processlist:

There are some lines at MysqlOnlineBackup_20140702.log where the NULL state can be seen:
"root localhost NULL Query 0 NULL FLUSH TABLES WITH READ LOCK"

Could that be a bug, if yes is it fixable, or what would be the possible workaround ?