iPad 6 randomly shutting down with a fully charged battery

Hi!

So my sibling has an iPad 6 which has recently presented with a lot of problems seemingly related to the battery. When the iPad is being used it'll randomly shut down and say it doesn't have enough battery despite being close to 75 - 80% battery. However, as soon as it's plugged in it works fine. But, when it's plugged in and reboots it says "100% Charged", then in the corner says something different, which may or may not be the current charge of the iPad. So I'm not sure if the problem lies in a false reading which may turn the iPad off or if there is something wrong with the battery. Thank you for taking the time to read this, hope you have a great day.