Had the same problem, and it seems to be related to an errant app.
In my case this was the "Automatic" car OBD connected app.
Wow, thanks for that, Craig! I've been struggling with battery consumption on my 5X. I noticed that Bluetooth was always at the top of the battery use list, sometimes over 20%. I decided to forgo Bluetooth as an experiment, but within seconds of turning it off, it was back on. As soon as I uninstalled the Automatic app, Bluetooth would stay off when turned off.
I also have Fitbit installed, but it doesn't seem to keep Bluetooth turned on.
Now after I get a day's measure of battery use with Bluetooth off, I'll pair my Zenwatch 2 again to see if it also abuses Bluetooth. I was on the verge of selling the smartwatch, blaming the phone's poor battery endurance on it. Hopefully, I'll be able to enjoy a full day of phone battery while using the watch.
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