Hi KevinSWilliams,
I am facing the same issue like you and tested everything similar to as you did, in fact reseting the phone and being an average RF semiconductor engineer almost every Technical and software/app based test etc. etc. but it doesn’t fix this.
After reading many complaint and reviews on different websites it seems like Apple Technical team failed to find the cause of the problem from a decade. And, I can see how efficiently they hire talents and support staff after talking to there technical representatives.
Apple really need to learn to hire talented professionals what they deserve to be in there company and shouldn’t just push everything on their core 20% Engineers. In fact they never check profiles on there job portals and uselessly pay a lot to a good amount of less qualified hires.
Anyways with some possible testing I performed I think the probable cause could be
1) Bluetooth Receiver firmware. as iPhone fails to receive the signal -( can be easily fixed by iOS update if it’s just instruction protocol mismatch) or else could be transceiver design bug or IC Mask layout integrity issue.
And if it is a other hardware lacking :
2) defected Bluetooth ICs or Packaging of miniature RF antenna inside the BT-IC. Could be % Yield issue.
3) Packaging defect specific to this model design ( as this is almost identical to iPhone 5/5s design which had wifi / bluetooth problem too). The closely placed and routed Bluetooth IC and the Wifi chip and broadband signals on the motherboard causing interference. As I notice it disturb SIM signal reception when we enable BT during device search.
4) And lastly could be, the magnet of compass is near RF antenna, notice some tolerance of few percent in showing North. All those models which has Bluetooth issue also has variations and shows wrong North and gps location.
Companies like Qcom / TI / Brdcom could have put a dedicated team of paid engineers for doing this failure analysis even if it is a defect in a design by their own technical project team. Surprised ! What Apple was doing for the whole decade!!