Suggestion, disable the Wifi, and cellular connection on the iPhone and see if you have the reboot or shutdown. If not enable the WiFi with out cellular to see if it happens, if not try the cellular without WiFi.
This is just a troubleshooting procedure I would try.
I would second this suggestion as well. I recall there was a thread where somebody was having streaming issues (blanked out or dropped connection or reboot, I can’t remember which) with the Oticon OPN (or maybe the More? I can’t recall) when connected to a slow wifi network (in the tens of Mbps). But when that person switched to a verified faster wifi network (in the 100 Mbps range), they still had issues. I asked them if they were on the 2.4 GHz wifi and if yes, try to switch to the 5 GHz wifi connection. They did that (moved from 2.4 to 5 GHz connection) and it seemed to solve the problem for them.
The MFI streaming between the iPhone and the Oticon OPN is at 2.4 GHz range, so there is possibly interference with the 2.4 GHz MFI signals if you use the 2.4 GHz wifi connection. Not saying that it’s YOUR issue, but it’s a valid troubleshooting step to take.