Okay, that's a tricky one, but the behavior you're describing gives us some clues. Here's a breakdown of potential causes and what to check

Understanding GFCI Behavior
Ground Fault Detection

GFCIs trip when they detect an imbalance between the current flowing to the load (through the hot wire) and the current returning (through the neutral wire). This imbalance (typically a very small amount, like 5 milliamps) indicates current is leaking somewhere it shouldn't – usually to ground.

Cumulative Leakage

Even small amounts of leakage from multiple sources can add up to trip a GFCI. This is important because it might explain why things behave differently with different loads connected.

Possible Causes and Troubleshooting Steps1. Flagpole Lights Themselves (Most Likely Culprit)



LED Driver Fault

LED lights often have a driver (power supply) that converts AC to DC. These drivers can develop insulation faults or leaky components that allow current to leak to ground. This is very common, especially in outdoor fixtures exposed to moisture.

This is your prime suspect. Moisture Intrusion

Even if the drivers are okay, moisture can get into the light fixtures (especially at the connections or seals) and create a path to ground.

Wiring Issues within the Fixtures

Damaged insulation on wires inside the light fixture could be touching the metal casing.

Testing



Visual Inspection

Carefully examine the lights, looking for signs of water entry, corrosion, or damaged wiring.

Resistance Test (Power OFF!)

With the lights disconnected, use a multimeter to measure the resistance between each wire (hot and neutral) and the metal casing of the light fixture. You should have infinite resistance (open circuit). A low resistance (even a few megohms) indicates a potential leakage path. Make sure to use an appropriate multimeter rated for high voltage testing.

Isolate Each Light

Since you noticed a difference with the second light disconnected, individually test each light by connecting only one at a time. This might pinpoint which light is causing the issue.2.

UF Cable to the Lights (Second Most Likely)



Physical Damage

The UF cable could have been nicked or damaged during installation (e.g., by a shovel, rocks, or roots). This can expose the conductors to moisture or the surrounding earth, creating a ground fault. UF cable is tough, but not indestructible.

Connection Problems

Loose or corroded connections in the junction box can create leakage paths.

Testing



Visual Inspection

Examine the cable along its entire length for any signs of damage. Pay particular attention to where it enters the ground or passes through any structures.

Resistance Test (Power OFF!)

At the junction box (with the breaker OFF and the cable disconnected from the GFCI and lights), measure the resistance between each wire (hot, neutral, and ground) and the ground wire. Ideally, you should have infinite resistance between the hot/neutral and ground. A low resistance reading indicates a ground fault in the cable.

Megger Test (Advanced)

A megohmmeter ("megger") applies a high voltage DC current to test the insulation of the cable. This is the best way to detect subtle insulation breakdown. However, meggers are more expensive and require more expertise to use safely. You might consider calling an electrician if you suspect a cable fault and don't have experience with this.3.

Receptacle Issues (Less Likely, but Possible)



Faulty Receptacle

The receptacle itself could have internal damage or corrosion that's causing a ground fault.

Moisture

Moisture entering the receptacle box can create a leakage path.

Wiring Errors

Although you say it works fine when the lights are disconnected, double-check the wiring of the receptacle. Make sure the hot, neutral, and ground wires are connected to the correct terminals and that there are no stray strands of wire touching anything they shouldn't.

Testing



Visual Inspection

Check for cracks, corrosion, or loose connections on the receptacle.

Replace the Receptacle

A simple and inexpensive way to rule out the receptacle itself.4.

GFCI Breaker Itself (Least Likely)



Faulty Breaker

GFCI breakers can fail. However, it's less likely, especially if it's relatively new.

Testing



Test Button

Make sure the test button on the GFCI breaker trips the breaker. If it doesn't, the breaker is definitely faulty.

Swap with a Known Good GFCI

If you have another GFCI breaker of the same amperage available, you can temporarily swap it in to see if the problem goes away.

Important

Make sure the replacement breaker is the correct amperage for the circuit. If you’re not comfortable with this, call an electrician.

Why the Different Trip Times?

The varying trip times suggest that you have a combination of: A small, pre-existing ground fault in the lights. This might be from slight moisture or early degradation of the insulation. The addition of the receptacle load pushes the total leakage current over the GFCI's trip threshold (typically 5 milliamps). When both loads are connected, the cumulative leakage exceeds the trip threshold almost instantly.

Important Safety Notes



Always turn off the breaker before working on any electrical wiring! If you're not comfortable working with electricity, call a qualified electrician. Electrical work can be dangerous if not done properly. Use appropriate safety equipment, including insulated tools and eye protection.* Follow all local electrical codes.Troubleshooting Strategy1. Start with the Flagpole Lights

Given your description, the flagpole lights are the prime suspect. Disconnect them completely at the junction box. Does the receptacle now work reliably without tripping the GFCI? If so, you've isolated the problem to the lights.2.

Isolate and Test Each Light

If disconnecting the lights fixes the problem, connect only one light at a time to see if you can isolate the faulty one.3.

UF Cable Test

If the lights seem okay individually, test the UF cable as described above.4.

Receptacle Check

Inspect and potentially replace the receptacle.5.

GFCI Breaker: Only suspect the GFCI breaker as a last resort.Good luck, and please provide updates as you troubleshoot! This information will help narrow down the problem. Flag for review