Automounted NFS filesystems are not being automatically unmounted after the timeout period in RHEL 5.8 and higher

Issue

• In previous versions (RHEL5.6, for example), when the RPC request (execution of stat command etc.) is regularly (at shorter intervals than the timeout period) done to an NFS file system which is mounted automatically by autofs, the filesystem was still unmounted after the timeout period.
• However, it is not unmounted in RHEL5.8 on the same condition.
• Automounted NFS filesystems are not being automatically unmounted after the timeout period in RHEL 5.8 and higher
• Please explain me the following:

1) What caused this phenomenon?
2) Is there a method that works the same as the previous version in RHEL5.8?