I am having odd issue. Some details first. The issue is occuring on a VM with bridged networking, in addition the connection in question is also automatically connectiong to a VPN.

There is no issues on the host, but on the guest (Fedora 28) for some reason the connection has stopped autoconnecting on boot. If I rdp into the guest and issue the command nmcli con up, it will go up just fine. If I use the GUI, go in to edit the connection, then save and go out, it will go up just fine. connection.autoconnect is set to yes. I am not sure why it will not autoconnect.

The connection had worked fine for years. However, I needed to change the VPN connection. So I added a new VPN connection, took the connection down, deleted the old one. When I tried to bring the connection back up, it failed and I was kicked out of the session. I rebooted the VM and it would not autoconnect, but will work fine if I connect it manually.

Any help would be appreciated. Thank you.

Examine the NetworkManager logging. That should pinpoint where the issue lies. If necessary share the logging here (eg via https://pastebin.com/) for others to advise further.

FWIW, I just tested my VPN connection on my openSUSE (Leap 15.2, KDE Plasma) guest. To make it start automatically, there is an option in the underlying 'DHCP' connection to start the desired VPN connection. The VPN connection itself can't be configured to start automatically via its configuration. I'm not sure if this is a recent change or not. I recall a time when a dispatcher script was required to make this happen.

My DHCP connection has the following in it
The secondary (VPN) connection is referenced by the last entry in the '[connection]' section of the config fie/

The uuid of my L2TP connection is shown below...