connectivity results in limited when behind proxy

The connectivity for a connection results in limited when behind a proxy for NetworkManager.

$ sudo nmcli general
STATE CONNECTIVITY WIFI-HW WIFI WWAN-HW WWAN
connected (site only) limited enabled enabled enabled enabled

$ sudo nmcli networking connectivity check
limited

I have configured my proxy in both '/etc/environment' and via the GUI under 'Settings'->'Network'->'Network Proxy'->'Automatic'.

After this is set I can do an HTTP GET against the URL/URI you have configured in the '/usr/lib/NetworkManager/conf.d/20-connectivity-ubuntu.conf' file.

$ curl -I http://connectivity-check.ubuntu.com/
HTTP/1.1 204 No Content
Date: Thu, 03 May 2018 09:01:34 GMT
Server: Apache/2.4.18 (Ubuntu)
X-NetworkManager-Status: online
X-Cache: MISS from proxy.example.com
X-Cache-Lookup: MISS from proxy.example.com:3128
Via: 1.1 proxy.example.com (squid/3.3.8)
Connection: keep-alive

$ env|grep -i proxy
auto_proxy=http://proxy.example.com/auto
https_proxy=http://proxy.example.com:3128
http_proxy=http://proxy.example.com:3128

But network connectivity still fails to use the proxy.

I assume that the 'network-manager-config-connectivity-ubuntu' package is used for this feature after reading the following output.

$ dpkg -S /usr/lib/NetworkManager/conf.d/20-connectivity-ubuntu.conf
network-manager-config-connectivity-ubuntu: /usr/lib/NetworkManager/conf.d/20-connectivity-ubuntu.conf

If I remove the '/usr/lib/NetworkManager/conf.d/20-connectivity-ubuntu.conf' file the connectivity check succeeds or I don't get limited connectivity at least.

--
$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 18.04 LTS
Release: 18.04
Codename: bionic

$ uname -a
Linux lnxhplap2 4.15.0-20-generic #21-Ubuntu SMP Tue Apr 24 06:16:15 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

$ dpkg -l|grep network-manager
ii network-manager 1.10.6-2ubuntu1 amd64 network management framework (daemon and userspace tools)
ii network-manager-config-connectivity-ubuntu 1.10.6-2ubuntu1 all NetworkManager configuration to enable connectivity checking
ii network-manager-gnome 1.8.10-2ubuntu1 amd64 network management framework (GNOME frontend)
ii network-manager-pptp 1.2.6-1 amd64 network management framework (PPTP plugin core)
ii network-manager-pptp-gnome 1.2.6-1 amd64 network management framework (PPTP plugin GNOME GUI)
--

Note: I have used "proxy.example.com" as substitute for our proxy server in this bug report for privacy reasons.