As mentioned to the upstream NetworkManager 1.36.2 VPN routing bug: https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/issues/946
The routing issue when the "Use this connection only for resources on its network" IPv4 setting is enabled, no longer appears to occur with NetworkManager 1.37.2. As 1.37.2 is a developer release, I believe this issue will resolve itself once the Debian network-manager package is upgraded to whatever the next NetworkManager non-developer release will be.
