Comment # 8 on bug 1120087 from Neil Brown

Ok, that explains it - or most of it.

There is a dependencies set up between exported filesystems and the nfs server.
The intention is to make sure the nfs server doesn't start until those
filesystems are mounted, but as a side effect, the nfs server is stopped in the
filessytems get unmounted.  That isn't really ideal.  I'll see if I can find a
way to fix that.

Your real issue is why does the usb device disappear on suspend.
That is probably what you want as a default, because it is quite possible that
you might unplug the device and re-plug it while Linux is suspended.  So Linux
needs to assume that it has changed.

If your intention is to keep the USB drive plugged in, then we should be able
to tell Linux that.

See:

https://www.kernel.org/doc/html/v4.20/driver-api/usb/persist.html

you should be able to set the device to "persist" across suspend/resume.  That
should leave the filesystem mounted and nfsd running.