When bucket level retention is enabled, you will not be able to write via NFS.  This is because NFS first creates a zero-length object and appends to it.  As soon as the zero-length object is created, it goes into retention which prevents further data from being appended to the object.  You can only read data via NFS if bucket level retention is enabled.  As an alternative, can you specify retention at the object level?
I thought with retention, you can post new files but when deleted they don't disappear until the retention period has expired... is that right?  Whenever I have retention enabled on a bucket and I try writing files to it over NFS, I get:  tar: IMG_5672.jpg: Cannot open: Read-only file system tar: Exiting with failure status due to previous errors  Remove the retention setting from the bucket, and it works fine (it errors because tar is trying to change ownership, but I understand why that's an error in this context).
NFS can't write files when retention active