Sounds like your phone battery IMO. I have the same phone and it only drops 10%. Airplane mode cuts off all communication also so no live.
It's the actual app that's a process hog, when I'm not using the app normal use I can easily get 8 hours DJI go is the same way for my phantom3There were known issues with the 6S batteries, Apple were doing replacements for quite a while.
Not sure if it is still an option but you might want to take it into an Apple Store and see what they have to say about 'rapid battery depletion'