"battery threshold" don't know what this means.The phones were rebooting when they reached certain battery threshold and in lower temps (i.e. winter) conditions.
You don't have to explain this issue to me. I had iPhone at that time. I knew why my phone is rebooting at 20% at -10°C. Still better experience than significant performance decrease and overall iPhone slowdown throughout the whole battery charge. Right from the 100% battery charge.
"and in lower temps" nope. that's not a requirement for causing the phone to randomly reboot in general. aged batteries can randomly reboot in any temp.