iPhone 8 Plus bootlooping when below 50%

My sisters iPhone 8 Plus was working fine until she had very bad reception in a place where there usually is very good reception. She tried restarting the device phone but it instead went into a bootloop. When I took a look at it I couldn’t get it out of the bootloop and it was very difficult to even get to the recover screen. I then tried to charge it a bit and after some time it worked. I gave it back to my sister and she used it for about 20 minutes until it just died and started bootlooping again, I came to the conclusion that whenever the charge went below 50% the phone dies it enters a bootloop, the battery health is at 100% and I’m using official apple chargers to charge her phone. The only thing I can think of is that it rained pretty heavy a few days before this happend. Does anyone know why this is happening? What are my options moving forwards?