A NFS mounted filesystem appears to be umounted while still visible in /proc/mounts

Issue

• NFS mounted filesystems seem to get unmounted within 10 min to 2 hours after boot without abovious reason
• The filesystem is still shown in the output of the mount command
• We do not see any errors in the logs
• A directory listing on the mountpoint with ls shows the content of the local filesystem in stead of the NFS filesystem