I'm so glad I found this thread! I was afraid I'd have to buy a new phone or get my battery replaced.
My 7 Plus has suddenly started experiencing dramatic battery drains on iOS 14.4. I think it has to be how the phone is reading the battery rather than the actual charge. I had 46% at 20:31, did a hard shutdown and turned it back on and I had 77% at 20:32!
This morning my battery suddenly drained to 1% (for no reason) but had managed to charge to 62% in around 20 minutes. It also drained from 100% to 67% in one hour with only 5 minutes of screen time. My battery health is showing as 93%.
I really hope the next update fixes this as I live in a place prone to earthquakes and prefer to have a full battery as much as possible, just in case...
You have a defective battery, get it replaced. That's the classic behavior of iPhone 7 (Plus) family batteries, they were inherently bad on some units, I had one of those. Just get it replaced.