Why is my iPhone 8 turning off

Don’t know where you are getting this beta thing from…

Are you running the latest version of iOS, instead of an older version?

Take it to an Apple Store. To me, the battery definitely sounds degraded, no matter what it says in settings.

Did you check battery health?

It sounds like, your battery is super degraded (source: have experienced something along the lines of this), and you should either get it replaced from Apple, or just get a new phone.