Nested group membership not updated until reboot

Hi folks, I was wondering if this is known/expected behaviour...

The Mac local Admin group has an AD group added to it (used the following code).

dseditgroup -o edit -a "mydomainmygroup" -t group admin

I add a new user to the AD group - the user cannot SSH to the device.
If I reboot the device, then the user can indeed SSH.

Could someone explain why this happens? Purely educational.
Thanks!