Something is wrong there. It should take days if not WEEKS of standby to drain that much!My iPhone 8 dropped overnight from 98 to 54 percent. Never before have I experienced such a huge battery drain. I'll give it one more more day or two and report my results.
My SE is doing the same. 100% battery health.Also experiencing worse battery life since updating from 10 to 12 on my SE. My 6 has had no such issues. All the more odd since my SE was the one with amazing battery life. Weirdest part is that the battery percentage drops in batches. It'll be a 50% and then drop to 48 or 47% while I'm looking at it (not doing anything major, not using GPS or watching a video). I've been through plenty of updates and never seen this before.
When I did the update, I was charging but the battery percentage still dropped 10% and the SE was REALLY hot after. Battery health at 90%