iPhone 6s battery issues
A reasonable explanation would be that the battery of your iPhone has come of age. Try to access the battery health (completed charge cycles, charge holding capacity etc.) using a popular battery assessment tool called coconutBattery.

Batteries has a fixed number of charge cycles and post consuming them, the charge holding capacity deteriorates.

It is also advisable to keep the iOS software up-to-date. Having it up-to-date alleviates any software related glitches that may be causing trouble with the hardware/battery.

Post analysis, if you feel that the battery is showing signs of going kaput, you can choose to go for a replacement.