Most likely this is caused by all other groups are handled with change_auth but wheel is handled by change_nacm (int ietf-system), and sometimes when the admin user should be added to wheel, the user admin does not exist. It says sys-cli also fails, but it is added to the user anyway.

Dec 4 14:04:18 ix confd[3290]: addgroup: unknown user admin
Dec 4 14:04:18 ix confd[3290]: Failed giving user "admin" UNIX sys-cli permissions.
Dec 4 14:04:18 ix confd[3290]: Failed giving user "admin" UNIX wheel permissions.
Dec 4 14:04:18 ix confd[3290]: addgroup: unknown user admin

To reproduce this:
run the test: ietf_system/upgrade in a loop on virtual devices.