On iPhones that have dynamic island (assuming you have a 15 Pro), you no longer get a pop up prompt when battery hits 20% to turn on low power mode. And previous phones didn't turn on low power mode automatically, you still had to manually do it.
Instead you get a notification within dynamic island and you need to long press on it to turn on low power mode.
No need to reset settings or anything. This is just how it works now with dynamic island.
You can also set up an automation to automatically turn on low power mode when battery hits 20%.