Me too, similar situation: an "rsync -a" syncs the timestamps of the original files first and after a short while the mtime of the file switches to the time of copy. Falling back to NFS version 4.1 on the mount fixes the problem.
I can confirm I'm running into the same bug.
root@proxmox:~# pveversion
pve-manager/9.2.4/5e5ae681198514d4 (running kernel: 7.0.14-4-pve)
I was using rclone to sync some files to an NFS share hosted on my Proxmox node. After a sync, running a...
I have the same problem on Proxmox 9.2.4 with kernel 7.0.14-3-pve as NFS-Server and NFS-clients running 7.x kernels.
A workarround for me is to mount the filesystem with NFS version 4.1:
mount -o vers=4.1 SERVER:/DIR /mnt
I guess it is a kernel bug.
Many thanks for commenting on this! Until now there was a chance I did something wrong and hadn't found it yet. But if you experience the same problem, there maybe really is a problem. I will check out with vers=4.1 and come back with a result...