Why sysctl variables are not exposed to the container when enabling both network and user namespace in Red Hat Enterprise Linux 7 ?

Issue

• Why sysctl variables are not exposed to the container when enabling both network and user namespace in Red Hat Enterprise Linux 7 ?
  Example: This sysctl variable visible under user and net namespaces independently:

$ sudo unshare -n ls /proc/sys/net/ipv4/vs/conntrack 
/proc/sys/net/ipv4/vs/conntrack

$ sudo unshare -U ls /proc/sys/net/ipv4/vs/conntrack 
/proc/sys/net/ipv4/vs/conntrack

But, when under both namespaces at the same time, the variable not visible.

$ sudo unshare -Un ls /proc/sys/net/ipv4/vs/conntrack 
ls: cannot access /proc/sys/net/ipv4/vs/conntrack: No such file or directory