They may not have connected the battery properly, not reinstalled the shield over the battery connector, or damaged a part of the battery gas gauge circuit.
I know on some of the older models like the 8, x, xs, xs max, xr, and more, it's possible to knock off a filter near the battery connector for the batter gas gauge circuit. There's a bms board attached to the battery that will relay information back to the device such as cycle count, voltage, temperature, and more. Typically when gas gauge isn't communicating with the device it will lead to the issue you've described where the device will only read 1% and reboot every few minutes.