The previous article I mentioned that the Fuel Compensation for IAT was affecting my AFR readings. After more datalogging and observations, I found that the bigger culprit was the Ignition Compensation for IAT. Essentially it was retarding the ignition once the IAT sensor reads 51C and above, resulting in the leaning of the engine. The leaning did not reoccur once I made adjustments to the compensation values.
However, the best way is still to relocate the IAT sensor to the air intake pipings like in the EKs instead of the intake manifold.