Q: Why iPhone 6s when I charge the battery percentage does not show correct or battery percentage remains constant?

Try a forced restart: Simultaneously hold down the Home and On buttons until the device shuts down. Ignore the off slider if it appears. Once shut down is complete, if it doesn't restart on it own, turn the device back on using the On button. In some cases it also helps to double click the Home button and close all apps BEFORE doing the restart.

Is it possible that this information pertains to your issue?  If your iPhone 6s or iPhone 6s Plus battery percentage doesn't update - Apple Support

Check your settings.