This happened to my RMN2 and RMN4. Initially I thought is my screen protector warping, so didnt care. Then i realise its weird, took closer look. WAH. Both RMN2 and RMN4 i did try to change, both also without much success. I remember the RMN2 gave out a loud pop sound, so I just stop.
Lessons learn. Don't overcharge, let it drain till as low as possible then start charging. Don't bother to DIY change parts though it may look easy, since Xiaomi phones (excluding Mi series) aren't expensive to buy also.
My current F1 serving me well for 2 years already. Only complain is why I didnt get the 128GB version