In Gadgetbridge's Discovery and Pairing Options that you get to after pressing the + button to add a device, is Ignore bonded devices checked?
That probably wouldn't be helping matters if so, but it sounds like if GrapheneOS is doing something to stop the Gadgetbridge app running in the background, even if you can get it connected to the Bangle it's not going to be much use to you if it's shut down every time you leave the app...
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In Gadgetbridge's
Discovery and Pairing Options
that you get to after pressing the+
button to add a device, isIgnore bonded devices
checked?That probably wouldn't be helping matters if so, but it sounds like if GrapheneOS is doing something to stop the Gadgetbridge app running in the background, even if you can get it connected to the Bangle it's not going to be much use to you if it's shut down every time you leave the app...