Most likely it's not a malfunction, but the system re-indexing itself after the update. When Android gets flashed, it rescans all the files, rebuilds the databases, updates search indexes - all of this runs in the background and drains the battery for several days. On the surface it might not show up in your app list, but there's serious stuff happening in the background. Try leaving your phone on the charger overnight and let it just do its thing - often after 3-7 days the battery consumption normalizes.
If nothing's improved after a week, check what's actually eating your battery. Open up the battery usage history by app and process (in Android this is usually Settings → Battery or Battery → Usage history). See which app is the biggest culprit. Sometimes after an update one of the built-in apps starts glitching and draining the battery constantly - like Google Play Services or the system search.
Last thing to try - clear the system cache. It's safe and often helps. Either through the app settings or through recovery mode (usually Volume Down + Power). If nothing works, you're left with a factory reset, but that's a last resort.