I just upgraded from iPhone 8 to 12 (mini) and use a Dexcom G6. I find that 12 periodically disconnects and reconnects from the Bluetooth for no particular reason, even when it is in the same room. It can remain offline for up to a few hours, but otherwise (90% of the time) it is fine.
The iPhone 8 remains connected all the while as does the Dexcom receiver. I am giving the 8 to my son, but hate to give up a reliable mobile phone connection.
The Bluetooth connection discovered by the iPhone 12 is labeled DexcomTH. It frequently shows as ‘not connected’ even when the devices are communicating properly. More than once I have rebooted both the phone and the Bluetooth, to no apparent avail.
The iPhone 12 connects successfully to other Bluetooth devices, so I’ve not figured out why it is temperamental only with the G6.