NFS ID Mapping in Kubernetes

I am mounting an NFS volume in my Kubernetes container. If I use the default runtimeClass, it works as expected - The file ownership is mapped to the users in the container, ie:

drwxr-xr-x 2 admin admin 4096 Aug 28 12:50 ./
drwxr-xr-x 3 root  root  4096 Aug 28 12:49 ../
-rw-r--r-- 1 admin admin    0 Aug 27 16:55 test2
-rw-r--r-- 1 admin admin   15 Aug 27 16:57 test3
-rw-r--r-- 1 admin admin    0 Aug 27 16:20 testfile

If I change only the runtimeClass to sysbox-runc, the file ownership is not mapped - Everything is owned by nobody:nogroup:

-rw-r--r-- 1 nobody nogroup    0 Aug 27 16:20 testfile
-rw-r--r-- 1 nobody nogroup   15 Aug 27 16:57 test3
-rw-r--r-- 1 nobody nogroup    0 Aug 27 16:55 test2
drwxr-xr-x 3 root   root    4096 Aug 27 16:59 ..
drwxr-xr-x 2 nobody nogroup 4096 Aug 27 16:57 .

I'm running in GKE, and using the Ubuntu with containerd (ubuntu_containerd) node type as was suggested in the docs, the kernel is version 5.15.0-1061-gke and shiftfs appears to be installed. The documentation suggests that with this kernel version and shiftfs the ID mapping should work. Any ideas?

Start by reproducing the NFS mount with the default runtimeClass and sysbox-runc on the stated GKE Ubuntu/containerd node, then inspect the runtime's shiftfs and ID-mapping behavior. Done means determining why ownership becomes nobody:nogroup and documenting or implementing a verified fix, with the result checked against the shown ownership output.