I've been having this issue recently with my 2015 F55. 99% of the time auto stop/start works flawlessly, but occasionally it would stop like normal then when I let off the brake, it didn't restart. Had to put the car in park and use the ignition switch to restart. Then yesterday it happened 3 times in a single trip. Pretty unsafe for the car not to restart at a stoplight or stop sign! Obviously I'm turning auto stop/start off when I first get in the car for the time being, but I'd like to get to the bottom of it. Not throwing a CEL or anything.

 

Any luck finding the fix? How is the health of your battery?

 

Just to update in case anyone else has this problem: wound up being the battery. Was an odd presentation of bad battery in my experience. I could still rapidly turn the car on/off 5-10 times without it failing to turn over, also the voltage was never low, even on cranking. I took the battery out on the off chance that could be the problem and it failed a stress test. Replaced the battery and no issues since.