Definitely the phone app waking up, at the bottom right of my phone screen it just spins and spins, and eventually maybe it works without an airplane mode on/off cycle. My S5 phone is Android 6.0.1 .
My 2011 Challenger R/T never had this problem in 3 years with the key fob, I changed its battery once. Put keys with fob in my pocket at the start of the day, leave it there all day, take it out at the end of the day. Simple. Leave phone in car whenever I wanted to.
Model 3 phone-as-a-key? Got to have the phone available to open the car or manually unlock it, got to be ready to switch the phone to the other hand to dig into my pocket for the key card if needed, every time I stop at a store or other location, I HAVE to take the phone in with me so the car can lock, and even if the phone unlocks the door that does not mean I can start the car every time. It's okay when it works, but even when it works I still have to take the phone into stores and places with me to get the car locked, and I never had to do that before, and it is not convenient. So even under the best circumstances the phone-as-a-key is still a pain.