I suspect this is the same as the following upstream NetworkManager 1.36.2 routing bug: https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/issues/946
I assume you have enabled the "Use this connection only for resources on its network" checkbox in the VPN connection's IPv4 settings? In which case network-manager 1.36.2 doesn't appear to be adding any routes for the VPN connection like it does if the checkbox isn't enabled or did with earlier versions of NetworkManager.
