Alrighty, so I finally figured it out. Turns out it was an exhaust leak at the manifold gasket at Cylinder 3, which is directly in front of the upstream oxygen sensor.
I found some white specks in the manifold in the port for cylinder 3, and did a bit of research. Turns out that's caused by moisture, most commonly from a bad head gasket. But, after taking a look at the old gasket, I found the 3rd cylinder hole was sooty, while the other two weren't.
This allowed fresh air into the exhaust manifold directly into the O2 sensor, which made the ECU think the car was running lean. This caused the fuel trim to increase to compensate, making the engine actually run rich.
What's disappointing is that fixing the leak brought the fuel trim back to normal, which also eliminated the boost in power I got from running slightly rich. The long term trim is down to 0-2ish instead of 5-7.
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View my fuel log 2014 Mirage DE 1.2 manual: 49.2 mpg (US) ... 20.9 km/L ... 4.8 L/100 km ... 59.1 mpg (Imp)