Many iPhone X and iPhone 8 plus user are reported issue with battery remaining indicator that Battery usage and standby are always shown the same. It’s just a glitch in the readings as the iPhone say usage was almost 17 hours and I just hit 20%, which does not seems to be realistic.
Battery life isn't really being affected at all. Just only get inaccurate readings with standby and usage both being the same. On iPhone with iOS 11.2 there not issue with the battery life but it's just incorrect readings. The problem is only reported when user upgraded their iPhone to latest iOS 11.2
Below are few fixed that you can try to fix the wrong battery usage bug in iPhone:
Fix 1: Reset Network Settings
Please do a network settings reset and it should immediately solved the problem. In case the issue occurs again you are requested to perform battery calibration
Fix 2: Calibrate battery
just ran iPhone down till it battery drains completely and shut off and let it completely charge. Now after a restart the issue should be fixed.
,Hopefully above steps fixed the issue for you but this is surely as this could be a software glitch. Apple is aware of the issue and hopefully will release a fix in upcoming iOS releases.
So it is recommended to update to latest iOS released by Apple for permanent fix