When you install a new version of iOS it takes a while for it to 'bed in'. It has to do various tasks in the background to get the device all updated properly. Some indexes are wiped and have to be recreated, for example. This takes battery power. This has been true for years.
You said: I made sure all my apps where closed and at the end of day my only way to slow the leak was to put it in battery saving mode.
Please don't do this. When you leave an app and go to the Home Screen, the app has about five seconds to finish what it needs to do, then it gets suspended. It uses zero power in the background. In fact, swiping those apps up out of the app switcher screen will cause you to use more power because the apps have to do more stuff when they start when you next launch them. It is a myth that apps use battery in the background.
And finally, if your iPhone is on a beta version of iOS then yes, this is the right place to ask for help. However, you've only said that your other devices are on betas, so you're probably in the wrong place. Try the Apple Support Forums. Thanks.