I’m on Graphene too and I once read that the modems used in Pixel phones are pretty trashy and drain a ton of battery. If I filter my battery usage by system, it shows that network standby almost consumes half of my battery charge (~46%).
Maybe the other guy here is right and it’s a VPN issue (too)? idk. Anyway, I didn’t really found a solution for this except leaving my phone on airplane mode at night.