16-gauge extension cords are not for space heaters.

Your cord is too thin for that load over that distance. A 1500W heater draws about 12.5 amps. A 50-foot 16-gauge cord has enough resistance that voltage drops significantly, which causes the cord to heat up — especially at the plug connection. More current + more resistance = more heat. The lamp draws under 1 amp, so it’s fine.

This isn’t a defect. You’re asking a wire designed for light-duty use (lamps, phone chargers, maybe a fan) to carry near its maximum rated current over a long run. The National Electrical Code says 16-gauge is only rated for about 10 amps at 50 feet for a single device. You’re exceeding that.

Save yourself a fire hazard. Use a 12-gauge cord for space heaters, or better yet, plug the heater directly into a wall outlet. Extension cords and space heaters do not mix.