Why does unmounting the filesystem returns error "device is busy" when lsof does not show any files held open in the memory?

Issue

• After stopping oracle 12.1.0 on any node in the rac cluster, /oracle can only be unmounted by issuing umount -l /oracle although lsof and fuser do not show any processes accessing /oracle.
• Once umount -l /oracle has been issued one single time, further starts & stops of oracle result in expected behaviour, e.g. umount /oracle is successful.
• nfs related services are not running.