I am so sorry about that! Did you get this phone at a store or over the phone? If you got it at a store less than 14 days ago, they can replace it there. If it was over the phone, we can replace it under warranty if you Contact Us.
I've gotten pretty poor performance on the T-Mobile Oneplus 6t fingerprint reader as well. It works great for me when I reset the fingerprint data but then pretty much treats me like a different person after an hour or so
My fingerprint died yesterday! If the only solution is to format only for it to die again!