Hi Andy,
there is nothing to do with the fact if there is an original battery or not. The guy from Apple has told you that, because it is simpler to say that and to move the problem to not original battery. They are mostly not engineers there and not experts at all.
I have new battery now. But I replaced this battery by my self. And I also have this notification about non original battery. But I don’t have this problem anymore.
The Apple supervisor (manager) from Frankfurt (Germany), told me that my problem is 100% the hardware(mainboard). I asked her (women) to replace at least the battery, she told me, my iPhone was opened before, so they can’t do that anymore for free. (I understand that, but why she didn’t admit to the software issue?)
I told her and an other guy, that there is a lot of cases with not charging iPhones after update to iOS 14 in Apple community. They answered me, they sometimes read these posts, but they don’t know if these posts are real or not. 🤣
To your problem:
Can you write exact what you did to try solving your problem, your steps?
Original battery (remove the notification)
You can buy a new original battery (with wrong serial number) and program it with special programer from china (JC V1S). It is easy, like two button clicks: „Read and Write“. You will need for that your old battery and your new original battery.
Maybe your old battery is damaged in some way. Maybe, because of high current consumption, there are some irreversible chemical processes happened. We don’t know that.
My current battery:
Original Apple but with wrong serial number.


Sorry for my english. I learn it now. 🤗