Hi!

You have nothing to worry about, it works as designed.
Some modern distros require new kernel version and do check the version during their work,
so we backport kernel features which those distros require from newer kernels
(so appropriate apps can use those features)
and fake "uname" output for those containers.

Actual kernel version reported is configured in templates config, for example:

# cat /vz/template/ubuntu/16.04/x86_64/config/os/default/osrelease
4.4.0

BTW, it was the same for OpenVZ 6.

Hope that helps.