Happens because in 3.2.x versions, there was no concept of gfid based backend, and with 3.3.0, we needed gfid backend. With FUSE mounts we don't have issues as 'lookup()' calls internally handle creating of missing gfids. But on NFS we do have to heal it ourself.
On NFS, we do experience a short duration of missing files till the self-heal daemon heals all the files.
Workaround (if any):
Run a full filesystem crawl after the migration on top of FUSE mount.
Files will appear on NFS mount also.