Know this has been years. One thing I've been doing lately is to convert mounts to an automount method using x-systemd.automount,x-systemd.idle-timeout=2min in fstab entries. These options are available in most recent Linux versions, and cause the client to unmount NFS if it has not been used in 2 minutes. Have seen this greatly reduce hung mounts during network maintenance/outages, unless the mount is actively being used during interruption. Doesn't truly fix, but has reduced needed cleanup work.
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