Static route problem

[Tadas]

Hello lists.
I have strange issue with KDE NetworkManager on Debian Unstable
(Stretch).
I'm using openvpn connection, and routes from OpenVPN server are not
pushed into NetworkManager. 

So I've tried configuring static route. The problem is that
NetworkManager ignores routes with 255.255.240.0 netmask. If I use
255.255.224.0 or 255.255.248.0 masks - static routes are created as
soon as i connect to vpn server. But it fails to create a route with
255.255.240.0 mask.
Any ideas?
Thank you.