The reason I suspect it's a closed loop issue is because the symptoms only appear once the engine is fully warm. Which as we all know is when the ecu switches to closed loop. Open loop is just for warm up. Hence why it runs rich.
Now the o2 sensor should be constantly changing if the car is running right. In a situation where your ecu is constantly adjusting fuel trim causing it to read that way. But I there is an issue causing the engine to run lean or run rich it will not flip back and forth regardless. It will simply display information from what it sees.
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