Issue with apps and acl

Hello all

I am running scale 24.10.2.

I’m using rsync for all my backups from all my devices.
I have 2 truenas scale, one primary running as a vm on proxmox and one is my backup server on bare metal.
A few months agon i had to rebuild my main server due to stupidity on my part.

Both server are configured exactly the same.

My backups run automatically every saturday afternoon and i have 6 datasets

My issue

There is not one week where my backup run smoothly. One rsync task or more will present a fault (23). the ACL of all datasets are configures using posix permission which i modified by adding some users.

I open the ace for that dataset (i change nothing). I just click on apply owner and apply group the i click to apply permissions recursively confirm and save.
Both server have the same datasets with the same acls
I run the same rsync task and it works

Have i missed a setting?
how can i stop this from happening?
Thank you in advance for your assistance