Using LittleSnitch 4.0.3 (and also tested with nightly build 5098 of 4.0.4) after successfully connecting to an IKEv2 VPN using the native MacOS VPN connection, no traffic is routeable. Pings are denied (not dropped) and traceroute fails at the first hop.
I rebooted in permissive mode and initially (first connection to the VPN) traffic routes fine, but after disconnecting the VPN and reconnecing again the traffic will no longer route. UDP/TCP traffic all appear to be blocked. Turning off the network filter makes no difference.
Uninstalling LittleSnitch restores normal functionality and the VPN works as it should - all traffic routes as expected.
I haven't found anything useful in console logs.