There are some tiny components next to the battery connector that allow the battery to communicate with the CPU about charge level. When these are accidentally dislodged during a repair, it will cause these exact symptoms.
I would open up the phone and closely inspect this area because they may have been partially damaged or dislodged during the repair and only came fully disconnected recently following a drop, for example.
Check the components highlighted in red in the picture below.
[image|2229054]