Check the HPHONE_TO_CODEC_DET_CON line. The expected value should be 0L.
If the reading shows 0.000, it indicates a short circuit in that line, which triggers headset mode.
Inspect capacitor C1816 for any signs of corrosion. If it’s clean, try removing resistor R1801 as a test. However, note that removing R1801 will disable the headset function.
If the issue persists even after checking these components, the problem likely lies in the audio IC U0900 (IC 338S1201).