Battery Drain and Sudden Shutdown Issue – iPhone 13 IOS 18.6

From the night of 24th August, my iPhone 13 IOS 18.6 started facing serious battery issues. I have already followed all the steps mentioned in Apple Support (iPhone battery and performance - Apple Support), but even after 7 days, the problem continues.

• Even when mobile data and Wi-Fi are turned off, the battery drains very quickly.
• For example, if my battery shows 80%, and I make a call or turn on mobile data, within 1–2 minutes the phone switches off automatically.
• After restarting, the battery percentage suddenly drops to around 10%.
• If I use it again (call or data on), the phone switches off once more, and after restarting, the battery percentage jumps back to around 75–78%.
• Last week, the Battery Maximum Capacity showed 65%, and now it has dropped to 64%.
• I am not sure what to do next, as the phone has become unreliable with frequent shutdowns.
• While typing this issue also my iphone 13 got 5-6 times frequent shutdowns and on.