Why is my iPad turning of even if my battery isn't low?

Updating iOS and your problem may just be coincidental. Was the battery performance decreasing recently, before the update? Was the battery ever replaced? If not, then the battery is already 5 years old and may simply be at the end of it’s life.

When troubleshooting charging issues, I would do the following (in order):

1. Always try another Lightning cable and charger, preferably Apple original or MFi certified units.
2. The Lightning cable 8-pin connector should be sitting flush with the housing and completely inserted. If it is not, there could be lint/dust/debris inside the port impeding a proper connection. You can clean it out with a sharp toothpick, fine point tweezers or a dental pick. Just be careful not to damage the pins inside the Lightning Port.
3. Change the battery. This is not trivial with an iPad however. The battery is the weakest link in the entire device and certainly for charger related issues. Use a battery utility, such as coconutBattery (for Mac) or 3uTools (for Windows) to test the battery prior to removal. Anything less than 70% of design capacity will require replacement.
4. If the battery is in good condition, you can use a USB Ammeter to determine if the phone is really drawing current when it says it's charging. If it isn't drawing current, then it could be the charge port.
5. If a new battery and charge port still don't resolve the issue, then you either have faulty replacement parts (possible) or a logic board issue. This can be repaired but requires micro-soldering.