I'm happy it works flawless for you man, and I'm sure on official Lineage builds which are as close as possible to AOSP things work exactly as you say.

I have an unofficial Lineage 18 ROM patched to hell to work with my old phone. All I can do is tell you what I see, and what I see is that when my phone tries to play a 720p or higher video, with an impossibly high bit-rate for the phone, the phone starts to aggressively background-kill apps, and that includes my VPN.

Again, happy it works for you, and I agree that in principle the default route should point to nothing if the VPN dies. On my device, when the virtual network device of the VPN goes down, it drops to the default network and finds another gateway.