One last thing to try... are you using eSIM or a physical SIM? Try swapping that out and then switch back to the original. You'll need to power off the phone and power on each time . I've seen weird things get resolved just by swapping the SIM card, not specific to iPhone or Google Fi. My guess is... provisioning at the carrier (T-Mobile) happens during boot, and something gets borked?

Of course, if you're using eSIM, then the next step might be to ask Google Fi agent to consider sending a physical SIM. On Android, there are ways to enable boot and cellular diagnostics logging per phone make/model. But AFAIK you can't do that on iPhone without tech ninja, a hard cable, and a diagnostic app like Xcode. But I haven't touched on that in years... from a previous lifetime.