Why does my phone restart at 20% while charging?

For context, I’ve had this Samsung A20 (not the A20e) for about 2 o 3 years now. It’s never had charging problems and the battery lasted a day and a half, sometimes more depending on usage. Lately it’s been rebooting every time I leave it charging and it hits 20% with the screen on. Never seen it happen while its not being used. It doesn’t have full storage, and it’s happened even while I only run whatsapp. I haven’t noticed it overheat either. I have worked fixing phones before, but I have never seen this problem on any phone. (And yes, this phone is mine). Would I need to replace anything on the phone?