Much like others, I’m surprised to hear about the battery life you’re experiencing. I’ve been amazed by the battery of my 11PM, even compared to my old Plus iPhones and a 3 month old iPhone X (AppleCare replacement). The 11PM is the first iPhone which has ever got me through a day with quite a lot of use. Though only after I set it up as new, when I restored from a backup it was giving me gyp, but not anymore.
It’s been quite liberating not having to continually top-up my phone, or having to carry around a battery pack for a change.
Maybe try restoring and setting up as new, if that doesn’t improve matters, I’d definitely have words with AppleCare, you should be seeing some improvement.