The problem is definitely in the gas gauge line between the logic board and the battery.
This could be a failed battery—and if there were no prior repairs, this is most likely.
The most common cause if the phone has been opened, is a missing FL 11 on the logic board. The FL 11 is a tiny component that commonly gets knocked off when people connect or disconnect the battery.
There is also a chance that you could have a secondary defect in the U2 charging IC, which is sporadically charging the battery—but this is secondary to your gas gauge line defect. It's also possible that there's a secondary defect in the charge port itself.
Apple diagnostics won't help you—they will just show how many amps the phone is consuming while charging, which won't help you to narrow down between these possibilities.
Your troubleshooting process should look like this:
1.) Replace the battery.
2.) If the problem isn't solved, look for the FL 11 on the logic board and send the phone out for repair if it's missing.
3.) If this doesn't resolve the problem, replace the dock connector.
4.) If the problem still isn't solved, send the phone out for a U2 IC chip replacement on the logic board.