When my SE1 battery was giving trouble, which started in the mid- to high-eighties percent Battery Health, the phone assured me that the condition was good enough to support normal function. Finally, with random power-offs and high drain in normal use - in other words, hopelessly incapable of supporting normal function - I replaced it myself and all the problems were fixed.

Battery life is not as good as when it was new, but I don’t know why that is. I think it’s a common experience, though. Ageing components? Running later versions? Who knows.

The fact that mine is good proves that the phone is still viable, so I’m guessing that something is wrong with yours - perhaps a bad battery.

Can you try a different charging lead? That may be the problem.

Do you mean that you bought it five months ago and immediately had the battery replaced? “Literally just bought the phone” doesn’t mean 5mo ago.

Batteries is batteries. Who knows why these things happen? But they do. I don’t check battery health any more. I’ve no idea what it is on my SE2, bought mid-2021. When the battery obviously affects performance adversely, I’ll replace it.