I am having this issue with NFS and digging in to it as well. Ill give the no cache drive a try and see if anything changes.
What I find odd in my case is that I can add the NFS share to my cluster, see it on both nodes, set up a backup job, run said backup job and everything works great. Leave it for a bit and come back and the storage on both nodes has a question mark and inaccessible. I have another NFS share mounted from another NAS and that has been solid.