Mefager Here are some suggestions:
- Keep a log (dates & times) of when Bluetooth works/doesn't, also Wi-Fi. Do they always shut off together? Is it always (or never) at home? Is Bluetooth connected to a device (such as a headset) when it shuts off?
- Though it would be inconvenient, try "Reset Bluetooth & Wi-Fi" in "Reset options".
- Though it would be very inconvenient, back up your data and "factory reset" GrapheneOS.
- Though it would be very very inconvenient, try flashing back to Google's stock OS.
Bluetooth and Wi-Fi shutting off without obvious cause -- especially if they both shut off at the same time -- often indicates hardware failure. But if it is not hardware failure, then one of the steps above that will reset or replace the software should work.