I’ve learned more about that bug in NFS.
As mentioned above, I used a workaround to add two lines to /etc/exports via the NFS Advanced Edit button in Rockstor. Both lines began with “/export/…”. They did not cause the problem of shading or occluding some of the directory contents. (See the posts above for all the details.)
Recently I tried exporting a third subdirectory using that same method and that bug returned, along with an additional corruption of the filesystem. Therefore, the workaround that I described above IS NOT SAFE, it simply happened to avoid encountering problems by chance.
The additional corruption of the filesystem is explained in detail in NFS Causes Corruption of Filesystem.
In conclusion, none of the workarounds as described in this thread are safe from the NFS bug. The use of any of them is not recommended.