I use my WH1000XM2 Bluetooth NC headphones with an iPhone 6s using iOS 13 software installed.

They had worked together very well for 1.5yrs and with different software updates on both devices. 

Today, for some unknown reason and for the first time, my headphones started to power down after 15-35 minutes usage. The headphones were charged to  medium level.
At first, I believed this was down to an iOS  for iPhone update, installed yesterday.  I did many things, including resetting the headphones to factory settings and repairing, but the problem continued.  
I also connected my headphones to my MacBook Air, but they too powered down after 30 minutes, proving that the iPhone software update was not the root cause for my headphones powering down issue. 

Does anyone have a fix for this annoying, power down problem?