@drugdealer
I have the same thing happen on my phone, but my phone always stays connected, it just doesn't automatically reconnect the next time
I don't know for sure, but it seems the setup wizard is the culprit. If that's the case, the developer team would need to change that or add a toggle to enable/disable mobile data.
For this kind of issue, I'd suggest opening an issue with them at https://github.com/GrapheneOS/os-issue-tracker/issues