I encountered the same problem just a few days ago with my iPhone 6 and iOS 10.3.3. It kind of hit without warning, and the only action I took that I can relate to the "always 100%" is changing the screen brightness. Otherwise I was using the phone as usual.
I tried the time thing (even though I was already automatically setting my time), restarting, pretty much everything short of flinging it out the window.
What fixed my phone was a tip I'd seen somewhere that said:
1) Put phone in Low Power Mode
2) Let it die
3) Plug it in to the charger and leave it plugged in for about a minute after the phone restarts
4) Let it die
5) Plug it in to the charger again, make sure it is still in Low Power Mode after it reboots and let it charge overnight
Like magic! My phone is now working as designed, with the battery indicator dropping normally!
Your mileage may differ. Void where prohibited by law. Try it, you may like it!