Hi
It could be a faulty battery
However
It could be that the battery needs recalibrated as new battery’s need to “learn” how to read their percentage properly
I have had loads of them that act like this and it’s usually that or the battery connector just needs reseated
I would try charging it up fully then draining it till flat a couple times to help recalibrate it and if the issue persists contact iFixit to see about getting a replacement or a refund
iFixit support is great so they will make it right:-)
Hopefully this helps
Any questions please ask
Thanks:-)