iPhone shuts off randomly after replacement battery

Hi, I have recently replaced a battery in a friend’s iPhone 6. He is reporting that the phone will sometimes just shut off at say 20% charge…

I am not sure how true this is, but he also says that it has been slightly better since doing an update?

The battery used was supposedly oem so that should not have affected it at all. However, when I was fitting the battery, I was having trouble getting the connector to sit in the slot flat? It seemed to be sticking up more at the side of the battery?

So, my question is - is it a faulty battery, a software issue or due to the incorrectly seated connector (it may have moved and become loose?)

If it is one of the latter two, then what can I do?